A Laser Diode(LD) end-pumped Nd:YLF quasi-continuous 351 nm ultraviolet laser with high efficiency and high peak power was demonstrated under acousto-optical Q-switched operation, by using extracavity frequency-doubled and frequency-tripled with two LiB3O5(LBO) crystals cling to the output mirror. An average output power at 351 nm of 450 mW was obtained at repetition rate of 1 kHz, pumped power of 14 W and output power of fundamental wave of 1.45 W. The optical-to-optical conversion efficiency is up to 31.04% and pulse width is 7.5 ns corresponding to the peak power as high as 60 kW.The beam quality is satisfactory.工系统方面的研究。