p-n junction single-gap solar cells are the earliest commercial and the most used structure. How to do faster and more accurate simulation and analysis by the given material, the structure of solar cell and the spectrum of light source is one of the most important researches. In this study, we proposed the numerical method for the equivalent circuit model of solar cells to do some quantitative analysis with the applied bias voltage, including the diffusion current, space charge recombination current, leakage current, and the voltage drop by the resistance.