Abstract: In frequency conversion systems of inertial confinement fusion, surface quality of KDP crystals with large dimension and low thickness is one of the key factors influencing the frequency conversion efficiency. For the third harmonic generation KDP crystals with 400 mm400 mm dimensions and with 45 departure angle from the horizontal plane, finite element models have been built up to analyze the effect of the fabrication errors on the additional surface variations of KDP crystals under different clamping modes, and the P-V and RMS values of the surface variations under different clamping modes and fabrication errors are given. The results show that the clamping modes and the fabrication errors are responsible for the additional surface variations, and the additional surface variation can be controlled by the clamping bars on the front even if the fabrication errors of KDP crystals and clamps exist.