A forced-turn-off mechanism of a passively and-switched pulse is proposed and demonstrated. To investigate the mechanism, we used rate equations and performed computer simulations. An auxiliary cavity for a passive Q-switch is inserted into a conventional passively and-switched Nd:YAG laser. Lasing of the auxiliary cavity restores the saturable loss and turns off the Q-switched pulse. The measured pulse profiles verified the turnoff, and the pulse duration could be controlled from 47.0 to 22.0 ns when we varied the auxiliary cavity length. Close agreement was obtained between the simulation and experimental results.