Combined with particle swarm optimization (PSO) algorithm, the parameter of photodiode was extracted from I-V characteristics curve based on the equivalent circuit which was obtained before and after irradiation on Si photodiode. Then the experimental law of equivalent parameter before and after being damaged was acquired. When the photodiode was damaged, the reverse saturation current was decreased and the series resistance was increased, and the shunt resistance was decreased. The damage mechanism was given qualitatively based on semiconductor physics theory. The decrease of the reverse saturation current was on account of decrease in doping concentration. The increase of series resistance was due to the decrease in doping concentration and carrier lifetime. The decrease of the shunt resistance was caused by the defects in the surface and internal of semiconductor.