diff --git a/core/src/lib.rs b/core/src/lib.rs
index 16ce753..3712640 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -1,4 +1,5 @@
 // Gobang core library — 纯游戏逻辑，零 GUI 依赖
 
 pub mod board;
+pub mod rules;
 pub mod types;
diff --git a/core/src/rules.rs b/core/src/rules.rs
new file mode 100644
index 0000000..228b3e5
--- /dev/null
+++ b/core/src/rules.rs
@@ -0,0 +1,197 @@
+use crate::board::Board;
+use crate::types::{CellState, Color, Position};
+
+pub fn is_forbidden(board: &Board, pos: Position, color: Color) -> bool {
+    if color == Color::White {
+        return false;
+    }
+    is_overline(board, pos, color)
+        || is_double_three(board, pos, color)
+        || is_double_four(board, pos, color)
+}
+
+fn is_overline(board: &Board, pos: Position, color: Color) -> bool {
+    let directions: [(isize, isize); 4] = [(0, 1), (1, 0), (1, 1), (1, -1)];
+    for (dx, dy) in directions {
+        let mut count = 1u32;
+        let mut nx = pos.x as isize + dx;
+        let mut ny = pos.y as isize + dy;
+        while let Some(cell) = get_cell(board, nx, ny) {
+            if cell == CellState::Occupied(color) {
+                count += 1;
+            } else {
+                break;
+            }
+            nx += dx;
+            ny += dy;
+        }
+        let mut nx = pos.x as isize - dx;
+        let mut ny = pos.y as isize - dy;
+        while let Some(cell) = get_cell(board, nx, ny) {
+            if cell == CellState::Occupied(color) {
+                count += 1;
+            } else {
+                break;
+            }
+            nx -= dx;
+            ny -= dy;
+        }
+        if count >= 6 {
+            return true;
+        }
+    }
+    false
+}
+
+fn is_double_three(board: &Board, pos: Position, color: Color) -> bool {
+    count_open_threes(board, pos, color) >= 2
+}
+
+fn is_double_four(board: &Board, pos: Position, color: Color) -> bool {
+    count_fours(board, pos, color) >= 2
+}
+
+fn count_open_threes(board: &Board, pos: Position, color: Color) -> u32 {
+    let directions: [(isize, isize); 4] = [(0, 1), (1, 0), (1, 1), (1, -1)];
+    let mut count = 0u32;
+    for (dx, dy) in directions {
+        if is_open_three_in_direction(board, pos, color, dx, dy) {
+            count += 1;
+        }
+    }
+    count
+}
+
+fn count_fours(board: &Board, pos: Position, color: Color) -> u32 {
+    let directions: [(isize, isize); 4] = [(0, 1), (1, 0), (1, 1), (1, -1)];
+    let mut count = 0u32;
+    for (dx, dy) in directions {
+        if is_four_in_direction(board, pos, color, dx, dy) {
+            count += 1;
+        }
+    }
+    count
+}
+
+fn is_open_three_in_direction(
+    board: &Board,
+    pos: Position,
+    color: Color,
+    dx: isize,
+    dy: isize,
+) -> bool {
+    let (cnt, start_open, end_open) = scan_direction(board, pos, color, dx, dy);
+    cnt == 3 && start_open && end_open
+}
+
+fn is_four_in_direction(board: &Board, pos: Position, color: Color, dx: isize, dy: isize) -> bool {
+    let (cnt, _start_open, _end_open) = scan_direction(board, pos, color, dx, dy);
+    cnt == 4
+}
+
+fn scan_direction(
+    board: &Board,
+    pos: Position,
+    color: Color,
+    dx: isize,
+    dy: isize,
+) -> (u32, bool, bool) {
+    let mut count = 1u32;
+
+    let mut nx = pos.x as isize + dx;
+    let mut ny = pos.y as isize + dy;
+    while let Some(cell) = get_cell(board, nx, ny) {
+        if cell == CellState::Occupied(color) {
+            count += 1;
+        } else {
+            break;
+        }
+        nx += dx;
+        ny += dy;
+    }
+    let end_open = get_cell(board, nx, ny) == Some(CellState::Empty);
+
+    let mut nx = pos.x as isize - dx;
+    let mut ny = pos.y as isize - dy;
+    while let Some(cell) = get_cell(board, nx, ny) {
+        if cell == CellState::Occupied(color) {
+            count += 1;
+        } else {
+            break;
+        }
+        nx -= dx;
+        ny -= dy;
+    }
+    let start_open = get_cell(board, nx, ny) == Some(CellState::Empty);
+
+    (count, start_open, end_open)
+}
+
+fn get_cell(board: &Board, x: isize, y: isize) -> Option<CellState> {
+    if x < 0 || y < 0 || x as usize >= board.size || y as usize >= board.size {
+        return None;
+    }
+    Some(board.get(Position::new(x as usize, y as usize)))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::board::Board;
+    use crate::types::Position;
+
+    #[test]
+    fn test_double_three_forbidden() {
+        let board = Board::new(15);
+        let board = board.place(Position::new(7, 5), Color::Black).unwrap();
+        let board = board.place(Position::new(7, 6), Color::Black).unwrap();
+        let board = board.place(Position::new(5, 9), Color::Black).unwrap();
+        let board = board.place(Position::new(6, 8), Color::Black).unwrap();
+        assert!(is_forbidden(&board, Position::new(7, 7), Color::Black));
+    }
+
+    #[test]
+    fn test_double_four_forbidden() {
+        let board = Board::new(15);
+        let mut board = board;
+        // 水平方向: (7,4)(7,5)(7,6) 已落黑子, 在(7,7)落子形成活四 (4子)
+        board = board.place(Position::new(7, 4), Color::Black).unwrap();
+        board = board.place(Position::new(7, 5), Color::Black).unwrap();
+        board = board.place(Position::new(7, 6), Color::Black).unwrap();
+        // 对角线(1,1)方向: (4,4)(5,5)(6,6) 已落黑子, 在(7,7)形成另一个活四
+        board = board.place(Position::new(4, 4), Color::Black).unwrap();
+        board = board.place(Position::new(5, 5), Color::Black).unwrap();
+        board = board.place(Position::new(6, 6), Color::Black).unwrap();
+        assert!(is_forbidden(&board, Position::new(7, 7), Color::Black));
+    }
+
+    #[test]
+    fn test_overline_forbidden() {
+        let board = Board::new(15);
+        let mut board = board;
+        for y in 1..6 {
+            board = board.place(Position::new(7, y), Color::Black).unwrap();
+        }
+        let board = board.place(Position::new(7, 6), Color::Black).unwrap();
+        assert!(is_forbidden(&board, Position::new(7, 6), Color::Black));
+    }
+
+    #[test]
+    fn test_white_not_forbidden() {
+        let board = Board::new(15);
+        let mut board = board;
+        for y in 1..6 {
+            board = board.place(Position::new(7, y), Color::White).unwrap();
+        }
+        let board = board.place(Position::new(7, 6), Color::White).unwrap();
+        assert!(!is_forbidden(&board, Position::new(7, 6), Color::White));
+    }
+
+    #[test]
+    fn test_normal_move_not_forbidden() {
+        let board = Board::new(15);
+        let board = board.place(Position::new(7, 7), Color::Black).unwrap();
+        let board = board.place(Position::new(7, 8), Color::Black).unwrap();
+        assert!(!is_forbidden(&board, Position::new(7, 9), Color::Black));
+    }
+}