By measuring the transmitted wavefront of crystals with orthogonal polarization interferometry(OPI), the model for determining the deflection angle of KDP crystals is established, and the theory is further verified to be correct and efficient by experiments. Under the stable conditions of probe laser and working temperature, the variation trend of prediction values is consistent with that of measurement results, and the difference is within 10.0 μrad. Furthermore, the factors affecting the measurement precision are analyzed and the differences between the theory and measurement results are pointed out. The model of the optimum deflection angles of full aperture for KDP crystals is established according to the single point model. In addition, the influence of crystal refractive-index non-uniformity on the optimum deflection angles of full aperture for crystals and conversion efficiency is also simulated numerically. The results obtained in this paper provide theoretic guidances for accurate prediction of KDP crystals in the engineering project.