In this study, a simplified one-dimensional heat transfer model is developed to assess the thermal effects of green roofs at the design stage. Instead of using temperature at the bottom of a green roof as the lower boundary condition, the equations for heat conduction through the surface and bottom of the substrate are adopted. Information required for the proposed model includes meteorological data and the thermal properties of the green roof. The model consists of two parts: the heat balance of the plant layer and substrate surface, and the conduction of heat through the interface of the substrate bottom. To verify the model, a green roof experiment is designed to collect data. Results show high consistency between the calculated and observed data, indicating the capability of the model in estimating temperature and heat flux of a green roof. In addition, the two thermal conditions, with and without a green roof, are analyzed for comparison. Significant differences of temperature and heat flux in summer are 14.5°C and 302.64W/m2 for the cool-down effect, and 4.90°C and 93.12W/m2 for the insulation effect, respectively, demonstrating the marked effects of the green roof in regulating temperature and heat flux.