Abstract: Rate equations of passively Q-switched lasers were numerically calculated. The optimization of thermally bonded passively Q-switched lasers was discussed. The effect of key parameters of passively Q-switched lasers such as the initial transmission of the saturable absorber, the reflection of the output mirror, the pump power was got by numerical simulation. Output mirror transmission can be optimized to get the maximized output power and pulse energy. Decreasing initial transmission of the saturable absorber can cause pulse energy increase, pulse duration decrease and the threshold pump power increase respectively. The repeat rate and output power increase with pump power approximately linearly, and can shorten the pulse duration while increase the pump power. Theoretic calculation results