Based on a circuit model of a photovoltaic (PV) cell with an extension term considering avalanche effect, the model of PV module with partial shading is established for the analysis of shadow effect. The validity of the model is verified with outdoor experimental test under real solar radiation condition. The output characteristics of one super cell at reverse bias-voltage, the I-V characteristics, P-V characteristics and yield of the serial module under different shadow block are analyzed by use of the model. The results indicate that the maximum output power drops 40% when the serial module has 40% partial shading. The output power of the module declines rapidly with shading increasing of one super cell. The maximum power decrease 7.36%, 25.81% and 97.94% respectively when each cell is shaded 1/5, 2/5 and the whole.