Theoretical modeling has been used to discuss advantages and disadvantages among the squash fixed mode, the silica fixed mode and the complete periphery clamping mode in this paper. Besides, numerical calculation has been used to analyze the influence on KDP surface and conversion efficiency. The results show that the conversion efficiency reduction of Gaussian beam is less than that of plane wave caused by surface deformation. when the intensity of fundamental frequency laser is up to 6 GW·cm-2, the conversion efficiencies reduce 7.5%, 9.0% and 7.2% to plane wave, respectively; and the conversion efficiencies reduce 1.3%, 1.0% and 1.5% to Gaussian beam, respectively.