Abstract: In order to achieve the ultraviolet pulsed laser with the high power and high frequency, the fourth harmonic 266nm in LiB₃O₅ (LBO)crystal was generated by frequency mixing of the fundamental(1064nm) and third harmonic (355nm) of electro-optical Q-switched laser, and experiment verification was carried out. Deep ultraviolet (UV) output power of 2.5W at 266nm with the repetition rate at 20kHz and 12.5% infrared(IR)-to-UV conversion efficiency were achieved. The result show that the pulse laser has achieved a large average output power at high repetition frequency by using LBO crystal.