A new kind of clamping scheme presented could make the crystal surface optimized, and numerical calculation is used to analyze the influence of crystal gravity sag on KDP surface and conversion efficiency by adopting the optimized scheme as well as routine schemes. The results show that optimized clamping scheme could improve the effects of KDP crystal gravity obviously. When the intensity of fundamental light is 5.5 GW/cm2, compared with routine schemes, the conversion efficiencies by adopting the optimized scheme increase 6.2%, 9.5% and 6.5%, respectively.