The research of clamping modes to large aperture KDP crystal and influence on surface figure by different clamping modes are done. Utilizing ANSYS the finite element model to simulate the obverse clamping and the flank clamping of KDP crystal, has been founded. KDP surface figure acted by flankly distributed load, equal flank two-point load and unequal two-point load is researched, and the conclusion shows that the whole load amout is a key factor to surface figure. KDP surface figures both in ideal state and in actual state have been discussed, and the conclusion that artifactitious errors for obverse clamp are a key factor has been obtained. The theoretical project has been acquired to minish the effect on artifactitious errors.