266 nm ultraviolet pulse laser is achieved by KTP crystal extra-cavity frequency doubling and BBO crystal extra-cavity frequency quadrupling of a laser in Nd∶YAG end-pumped by LD and passively Q-switched by Cr4+∶YAG.The basic principle of passively Q-switched is analyzed,and the influence of the initial transmission of the Cr4+∶YAG on single pulse energy of the 1064nm laser under the ideal condition is calculated and simulated.When the pump power is 4.8W,the output power of 5.63mW and single pulse energy of 0.5μJ at 266 nm laser is obtained.Certain progress of practicability of compact milliwatt levels 266nm laser has been made.