In recent years, most studies of green roof system are focused on short period measurement or modelling without considering long-term performance, however, the latter is concerned by many architects and engineers. And this paper is aimed to evaluate green roof's long-term thermal per-formance in Shanghai district based on a new model and analyze various factors' effect on thermal index. Green roof is usually divided into three big layers, namely plant layer, substrate layer and structure layer. The model developed in this paer not only considers the effect of wind velocity's vertical distribution on coupled hygrothermal transfer, but also involves multiple reflection of solar radiation between different layers. Based on the field measurement from July, 2014 to June, 2015, the model has a good agreement in most cases. And simulation results indicate that green roof has a better insulation effect than corresponding common roof both in summer and winter, and its equiv-alent thermal resistance is not a fixed value. Many factors shows siginiciant effect on green roof's long term thermal performance, such as LAI, soil's thermal conductivity, indoor temperature etc. According to the method in this paper, it is possible to have a quick comparison of thermal perfor-mance between green roof and common roof, and judge whether green roof system meets the insulation rule of local energy conservation.