Green roof is capable of significantly reducing rainwater volume and purifying rainwater quality. It has become one of the important strategies to solve urban floods. This study investigated the ratio of runoff retention, effluent process and effluent water quality by building pilot scale facilities. Results from the experiment confirmed that the ratio of runoff retention is accordance with a certain periodic formula; the total runoff volume, the amount of previous reduction and later reduction have different linear correlations with the thickness of substrate layer; and green roof is capable of significantly reducing the concentrations of TN and COD, but the concentrations of NH₄-N and TP in runoff from green roof are higher relatively. Finally, this study recommends that the optimal range of the thickness of substrate layer is 10-20 cm by integrating the two aspects of hydrology and water quality.