The output characteristics of a passively Q-switched Nd∶YAG/Cr4+∶YAG/YAG composite crystal laser at 1 064 nm was studied with diode-end-pumped system. The laser output characteristic parameters were compared among five output couplers with different transmittance, and between two Cr4+∶YAG crystals with initial transmittance of 84.1% and 90.6%. The experiment results show that the output parameters as average output power, pulse repetition frequency and pulse width could be optimized with transmittance of the output coupler. And the optimized transmittance also increased with the pump power. By the comparison of laser output characteristics between two crystals with different initial transmittance, the crystal having initial transmittance of 84.9% produced relatively low average power output, but its low pulse repetition frequency and narrow pulse width made the Q-switched laser peak power improved significantly. Under the incident pump power of 10.4 W, the maximum output power of 3.2 W was obtained using the output coupler with 30% transmittance. The calculated peak power was up to 6.3 kW as corresponding pulse width and pulse repetition frequency of 9.7 ns and 52 kHz, respectively.