Abstract: Considering the additional surface variation of large-aperture thin KDP crystal placed with the departure angle of 45 from the horizontal plane in inertial confinement fusion (ICF) systems, which caused by the fabrication errors of KDP crystal and its mounting system, a finite element analysis model has been built up for the KDP crystal based on the experimental measurement data of surface profile and its mounting system with fabrication errors of different types and sizes. The influence of the fabrication errors of KDP crystal and its mounting system on the additional surface variation of KDP crystal has been simulated and the P-V and RMS values of the crystal surface profiles have been given. It is shown that the fabrication error of the edges of KDP crystal and the concave fabrication error of support surface of mounting system are important reasons for relatively large additional surface variation, and the fabrication error of KDP crystal leads to nonuniform deformation. Whereas the convex and wavy fabrication errors of support surface and the random fabrication error of strip surface of the mounting system lead to relatively small additional surface variation. The additional surface variation caused by the random fabrication error of support surface is in between.