Based on the existing experiment, the heat flux characteristics of green roof in passive indoor thermal environment and thermal parameter suitable for evaluation of the thermal performance of green roof were analyzed. It is found that there is indoor critical temperature in acceptable indoor thermal environment range for green roof, in which there is no heat transfer from roof. Through the correlation analysis of internal surface heat flux of roof and the temperature difference between indoor and outdoor, it is shown that the indoor critical temperature is 1.5℃ lower than air temperature outside for green roof.