To study the internal mechanism of the photodiode nonlinear response to the high energy laser pulse, the influence of the space charge screening on the photoelectric response characteristic of the InGaAs pin photodiode was calculated under the high optical injection. The electrical field, distribution of carriers and electronhole mobility in the depletion region were calculated respectively. The simulated results indicate that when the applied bias field is low or the laser pulse energy is high, the electrical field in the depletion region is suppressed and the driftdiffusion velocity is reduced to the unsaturated state. Therefore, the separation and recombination of the holeelectron is slowed down, lots of the photocarriers is in the depletion region, which leads to the nonlinear response due to the space charge screening. In the experiment, the InGaAs pin photodiode′s response voltage pulse width is widening and the peak voltage is increasing nonlinearly with the increasing 20 ps laser pulse energy in the 5 V applied bias. The calculation of the photodiode internal mechanism is proved to be valid by the experimental phenomena.