The output characteristics of a LDA end-pumped Cr∶YAG passively Q-switched Nd:YAG laser are investigated. The influence of the transmissivity of the output coupling mirror (T=3%,8%,20%,30%,40%) on the average output power,pulse width,repetition frequency,energy per pulse and peak power was analyzed. The results show that,with specific laser parameters,output mirror transmissivity decides some output parameters of Cr∶YAG passively Q-switched Nd:YAG laser. When the transmissivity is 8%,the maximum output power and repetition frequency are obtained;and when the transmissivity is 20%,the narrowest pulse width,the highest peak power and the maximum energy per pulse are gotten.