Abstract: The factors attributed to the efficiency of the extra-cavity third-harmonic generation are analyzied.The maxium output power of 355nm UV laser is 330mW with the incident pumped power 17W after optimizing the small singal transmittance of the Cr4+:YAG,the transmittance of the output coupler at 1064nm,the focal length of the focusing lens,the nonlinear crystal and aligning the optical elements reasonably.The repetition rates,pusle width,single pulse energy of the pulsed 355nm laser is 28kHz,12ns,12J respectively.The overall light to light efficiency is 2%.