The numerical method is used to solve the rate equations when the stimulated state absorption of saturable absorber is considered. The passively Q-switched output pulse energy as a function of the output reflectivity for several initial transmissions of the absorber is demonstrated. The output parameter is also demonstrated when the laser operates at maxim output energy. From numerical results of rate equations the output energy is estimated,and the key parameters of the the Q-switched lasers such as initial transmissions of the absorber and the output reflectivity are optimized. Finally,a Q-switched Ruby laser experiment data are analysed to illustrated the optimized results of the parameters.