In this paper, a model of photovoltaic system under shading conditions (called as power law model) is proposed and studied. And, another model (called as four-parameter model) is used as a comparative study. Firstly, the results show that, the two models are roughlySequivalentsStoSthe experimental phenomena, although there are obvious errors between theoretical calculation and experimental data. The reason is that both models are obtained with simplicity. Secondly, it can be seen that both models are in good agreement with the experimental data when the cell is at low bias voltages, while the significant errors occur as the cell at high bias voltages. The reason is that, the cell can be regarded as a constant current source and load as the cell at the low bias, with the increase of voltage, the nonlinear effect of all kinds of solar cells is enhanced significantly which results in that the power generation model is not applicable. Thirdly, noting that the operating status of the each cell in the module can not be obtained by a simple analysis when the module is at non-uniform light intensity. In fact, whether the bypass diode is in a blocked state is a gradual process rather than a simple abrupt process. Finally, from root mean square error analysis, it shows that the four-parameter model is better than the power-law model. The reason may be that the four-parameter model takes into account the effect of photocurrent, reverse saturation current, ideality factor, series resistance.