Abstract: According to the spatial distribution data of the refractive index of KDP crystals obtained with orthogonal polarization interferometry (OPI), the internal detuning angle distributions have been calculated. Consequently, the optimization model and method of the best input angle of full aperture for doubler and tripler KDP crystals have been established. The variations of the input angle of doubler and tripler KDP crystals with different initial intensity of fundamental field have been analyzed and discussed. On this basis, the best input angles of the full aperture for measured KDP crystals have been optimized. The results show that doubler is sensitive to non-uniformity of refractive index while tripler is not for the relatively large values of RMS of internal detuning angle. Consequently, the method for determining the best input angle of the full aperture for doubler and tripler KDP crystals can be optimized. In practical applications, we optimize the best input angle of actually measured tripler assuming the refractive index of doubler is uniform at first. After that, the best input angle of actually measured doubler and tripler could be obtained through appropriately adjusting the input angles of doubler and tripler, respectively.