This paper presents a simplified photovoltaic array model to describe the photovoltaic system features under partially shaded conditions. Based on the proposed model, the peak power sliding phenomenon of photovoltaic generation is studied during the above mentioned system dynamic process. The paper analyzes the mechanism of the peak power sliding and the critical sliding conditions. Simulation cases are provided to verify the theoretical analysis. The simulation results also show the difference of dynamic characteristics between shaded photovoltaic system and non-shaded photovoltaic system.