A crossbow includes a supporting unit, a bow, a string-pulling unit, a trigger unit, a sight and a security unit. The security unit includes a lock, a lock-biasing spring and a pusher. The sight includes an unlocking window. The lock is movable in the sight corresponding to the unlocking window between a locking position and an unlocking position. In the unlocking position, the lock prevents the trigger unit from pivoting. The lock-biasing spring is inserted in the sight and adapted for pivoting the lock to the locking position. The pusher is connected to the string-pulling unit and hence movable relative to the supporting between two positions through the unlocking window. In the first position, the pusher keeps the lock in the unlocking position. In the second position, the pusher allows the lock-biasing spring to move the lock to the locking position.
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