Theoretical analysis about elements that influence pulse width of Q-switched laser was carried out. On the basis of rate equation, expressions of the relations among inversion density, photon density, and cavity loss under pulse stretching conditions were established. The cavity loss’s variation curve was plotted after numerical simulation when flat-topped laser pulse was obtained. In the experiment, proper structure of resonant cavity was designed, and by taking advantage of two thyratrons as high-voltage switches, the variation of voltage on KD*P can be controlled, so the loss can be controlled to make it satisfy the pulse stretching conditions. Finally, 420 ns Q-switched laser pulse was obtained.