Laser characteristics of acousto-optic Q-switched operation of 1319nm/1338nm dual-wavelength composite Nd:YAG laser were studied. Maximum output power of 5.77W was achieved in CW operation. Under Q-switched operation, the maximum peak power of 3.96kW and minimum pulse width of 65.6ns was obtained at repetition frequency of 20kHz with the duty ratio of 96%. The influence of the duration of the ultrasonic field acted on the Q-switch to the output characteristics of dual-wavelength composite Nd:YAG laser had been reported first time.