In order to optimize the design of laser resonant cavity, analytical solutions concerning Gaussian beam were derived in a three-mirror V-folded cavity by using the theory of optical propagation matrix. The cavity stability of the folded cavity laser was investigated with an equivalent simplified model. The influence of resonator parameters, such as folded mirror focus, cavity length and curvature radius, on the stability of Q-switched lasers were studied. The stable region′s boundary curve expression is obtained with the off-focus amount as a design parameter. According to theoretical analysis, a mechanically chopper-typed Q-switched CO2 pulse laser was taken as an application example. It was found that for one resonator with preset parameters, as long as the other resonator chose appropriate off-focus amount, the whole laser folded cavity was stable. Either of the resonator cavities can work with two stable regions, instead of previously supposed one. Appropriate stable position can be selected according to preset conditions. By adopting off-focus amount instead of factor g as the parameter, the stable region boundary and maladjusted error are derived directly, which is simple, straightforward and convenience for application.