A quick connector includes a connector body, a spring, a plurality of positioning balls, a push ring, a movable sleeve, a snap ring, and a locking structure. When a coupling is inserted into the connector body, the push ring is pushed the spring to press the positioning balls which press the coupling so that the coupling is locked onto the connector body. The movable sleeve is driven to return the push ring to release the coupling from the positioning balls. Thus, the movable sleeve is locked by the locking structure and cannot be moved any more so that the movable sleeve will not be driven inadvertently due to an external force and can press the positioning balls to lock the coupling so as to prevent the coupling from being detached from the connector body.