Green roofs (GRs) are widely employed in urban areas to mitigate the detrimental impacts by stormwater flooding. However, a simple yet physical method for assessing the hydrological performance of GR is still lacking. In this study, we first perform an analysis on the relationship between rainfall depth P and runoff amount R based on the water balance principle and discuss the different runoff generation mechanisms on GR. A field experiment is then conducted to examine the P-R relationship constructed in the previous analysis, which demonstrates good validity in estimating runoff generated on our GR. Numeric simulations via HYDRUS-1D are then carried out to investigate the runoff generation mechanism on GR as well as to examine the applicability of the proposed assessment method. The simulation results indicate that both the saturation and infiltration excess processes occurred on our GR; nonetheless, the infiltration-excess runoff accounts for only a small portion of the total runoff. This implies that the proposed linear model is applicable for estimating the runoff from GR even under heavy rainfall condition where the infiltration-excess mechanism is supposedly active.