With the establishing of a mathematical model of P-N junction,the difference between a practical P-N junction and its ideal model was analyzed. The volt-ampere properties of diodes and solar cells were simulated through the model. The influence of series and shunt to open-circuit voltage and short-circuit current of solar cells under certain illumination were demonstrated. With a simple model established by Matlab,the equivalent circuit of a solar cell was numerically analyzed and solved though a equation. At last,I-V properties of a Si solar cell were measured and compared to the I-V curve calculated by our model whose parameters were setting properly. The results show that the model established in this paper is consistent with the practical devices.