During the actual operation，photovoltaic (PV) module is affected by various environment factors such as ambient temperature, ambient humidity, solar irradiation, wind speed and their coupling effects. Different degrees and types of degradation occur, the performance degradation, especially the degradation of output power, is difficult to accurately assess. Based on Finite Element Method (FEM) and failure dynamics, a degradation model of output power is proposed, where environment factors and their coupling effects are considered. The degradation data of output power of PV module in LongYou power plant is obtained. The Step-Down Stress Accelerated Degradation Test (SDSADT) is carried out, and the actual degradation data of output power is obtained with the help of acceleration factor. The validity of the degradation model is verified by comparing the results assessed by the SDSADT with the results assessed by the model. Based on orthogonal experiment and degradation model, the influence degree of four environment factors on performance degradation of PV module and the sensitivity sequence of the four environment factors is explored. The results show the influence of ambient humidity on performance degradation is the most obvious, and solar irradiation has the least impact on performance degradation.