The nonlinearity of an In<sub>0.53</sub>Ga<sub>0.47</sub>As on InP p-i-n photodiode due to high-energy laser irradiation is simulated using quasi-three-dimensional iterative solutions to the drift-diffusion and Poisson equation in the presence of generation-recombination terms. Some necessary physics models used to simulate the realistic device are discussed, such as the recombination model and mobility model. The calculation results indicate that the peak amplitude of the voltage response to the laser increases sub-linearly and the relative contribution of the tail to the detector response is appreciably enhanced with the intensity of the laser increasing. With the different reverse bias decreasing, the photodiode response saturated voltage reduces, the nonlinear is more serious and the responsivity of the photodiode is decreasing with the same pulse energy. The space-charge-screening effect due to the carriers generated by the high optical injection restraining the internal electrical field is responsible for this nonlinearity. The experiment of the p-i-n photodiodes response to picosecond pulse laser is carried out. By measuring the response voltage and time of the InGaAs p-i-n photodiode, the results are verified the theoretical simulation.