Owing to the effect of clamp and gravity on the large aperture KDP crystal, additional surface error would be introduced in the frequency conversion system. The method of complete periphery clamp in KDP crystals has been presented，and the theoretical model simulating the situation that KDP crystals vertically mount has been analyzed. The results show that: symmetrical sustentation and even force make the crystal surface suitable for engineering application, and the requirement of physical experiment is able to be met when the force on the 4 nails of the above profile is less than 25 N under the condition of symmetrical sustentation and even force.