In final optics assembly of high-power solid-state laser, in order to improve the third harmonic generation (THG) efficiency, the method of both sides support for ultra-thin KDP crystals with large-aperture is adopted, and simulation model, is built using finite element analysis software. On this basis, the influence of parameters of clamping system including the number and length of support bars on the additional surface shape of KDP crystals, phase matching angle and THG efficiency are calculated and analyzed for the case of KDP crystals placed at different tilt angles. The results show that, when the number of support bars is four, the peak valley (PV) and root-mean-square (RMS) values of additional surface profile of ultra-thin KDP crystals with large-aperture, the mean value of crystal axis and THG efficiency almost keep unchanged with the length of support bars and an ideal THG efficiency is achieved. Consequently, the problems of repeating test on the quality of KDP crystals′ surface profile can be solved, and the time for engineering assembly and calibration can be reduced.