266 nm ultraviolet pulse laser is obtained by passively Q-switched Cr4+∶YAG，KTP crystal which are used for intra-cavity frequency doubling and BBO crystal which is used for extra-cavity frequency quadrupling.The laser is an LD end-pumped Nd∶YAG laser.By optimizing external resonator of the laser，the efficiency of the nonlinear frequency conversion is improved.When the pump power is 4.6 W，the 532 nm laser average output power is 154 mW.Compared with external cavity frequency doubling，the average power of 532 nm laser is improved by three times，single pulse energy and peak power are improved by 2 times.The output power of 266 nm laser is 3 mW.