A diode-pumped Nd: YAG acousto-optically (A-O)Q-switched laser at wavelength 946 nm formed with a simple plane-plane cavity has been demonstrated. The maximum average output power was 850 mW. The highest peak power was 531 W with the pulse repetition rate of 20 kHz and pulse width of 80 ns at the incident pump power of 19.5 W. Supported by Key National Science Foundation of Shandong Province(Z2003A01)