An intracavity frequencytripling of a diodeedgepumped passively Qswitched 1 064 nm Nd∶YAG laser is presented to generate high power 355 nm UV laser. Several essential technologies are put forward to achieve the best beam quality such as a volume grating to maintain quasi phase matching with narrow linewidth, optimization and thermal effects compensation for cavity. The task first takes advantage of volume grating and two CBO crystals as SHG and THG nonlinear optical conversion material to obtain high UV laser power output more than 4 W, light quality factor M2<2.5, meanwhile it is the first time to realize thermal effect using 45° Faraday rotator. The achievement is expected to be the popular light source system in lithography instead of conventional excimer laser.