A diode-pumped,Cr∶YAG passively Q-switched Nd∶YAG laser with an ultraviolet output at 266 nm is studied.Usinga LDA with 15 W,when the incident pump power was 12 W,the passively Q-switched average power infrared output power reached 2.2 W,and the period of the pulse was 40 μs,and the width of the pulse was 18 ns,and the peak power rose up to 4.9 kW.Taking KTP as the second harmonic generation crystal,532 nm green light average power was 850 mW;Taking BBO crystal for fourth harmonic generation outcavity,266 nm ultraviolet average output power was 215 mW,the efficiency of green to ultraviolet conversation was 25.2.%,and the efficiency of infared to ultraviolet conversation was 9.8%.