Based on E. Sano′s metal-semiconductor-metal photodetector (MSM-PD) model, an improved numerical model is presented. The model is built with parallel current sources and capacitance. The sum of excess electron and photon in absorption layer is studied, and the rate equations are solved. The capacitance is calculated, the nonlinear relationship between the dark current and the terminal voltage is derived, and the linear calculation of the dark current in conventional model is improved. The mathematical expression of the photoelectric current is presented. The simulation analysis by Matlab indicates the calculation is simplified with a higher precision. This model represents the change of photoelectric current under some bias and illumination conditions, and shows the transfer of photoelectron in devices. This model can also be used in measuring and simulating weak signals.