Abstract: In the field of inertial confinement fusion (ICF), UV laser can effectively improve the beam-target coupling efficiency and suppress the hydrodynamic instability of laser plasma interaction. Using large aperture KDP crystal or KD*P crystal to triple the frequency of laser created by high power solid-state laser facility (wavelength is 1 μm) is an effective way to obtain high power UV laser output. At present, most high power laser facilities take method of I/II type third harmonic generation. The phase matching condition is required in the third harmonic generation, and the conversion efficiency is easily influenced by the detuned angle of crystals. The quality of the return light spot of crystals’ surface can reflect the state of wavefront distortion, and then evaluate the degree of angle-detuning at the crystal. The factors which create detuned angle are analyzed and judged, within which the surface shape distortion of crystals (PV is more than 30 μm) caused by clamping and gravity is the main part. According to improving the surface shape of crystals (PV is less than 10 μm) in the experiment, the quality of the return light spot of crystals has been significantly improved, and then the conversion efficiency of third harmonic generation has an obvious increase.