The Middle and Lower Reaches of the Yangtze River (MLRYR) region, which has humid subtropical climate conditions and unique plum rain season, is characterized by a simultaneous high-frequency urban flooding and reduction in groundwater levels. Retrofitting the existing buildings into green roofs is a promising approach to combat urban flooding, especially for a densely developed city. Here, the application potential of the Green Roof System (GRS) and the Improved Green Roof System (IGRS) that designed to divert overflowing water from green roofs to recharge groundwater were analyzed in such a densely developed city, Nanchang, China. The performances of the GRS and the IGRS were evaluated using the United States Environmental Protection Agency (USEPA) Storm Water Management Model (SWMM). The simulation results show that in single precipitation events about 41-75% of precipitation could be retained in the GRS depending on precipitation intensity. In 10- and 100-yr precipitation events, the flooding volumes in the GRS region are 82% and 28% less than those of Traditional Roof System (TRS), respectively. For the first time, the influence of GRS on the hydraulic condition of CSS / SWS (Combined Sewage System / Storm Water System) is analyzed, which is a direct reflection of the effect of GRS on alleviating urban flooding. Recognizing the limitation of SWMM, five methods have been used to comprehensively analyze the evapotranspiration process of GRS. The evapotranspiration of the GRS retained water could account for 39% of annual precipitation. Although the IGRS could lead to a higher immediate flood loading (about 20-27%) than the GRS, it could divert more precipitation (more than 10% of the annual precipitation) into the greenbelts, thus significantly increase groundwater recharge. We may conclude that the widespread implementation of both the GRS and the IGRS in Nanchang and other densely developed cities in the MLRYR region could significantly reduce surface and peak runoff rates. In particular, the IGRS can provide more hydrological benefits than the GRS under the same climate conditions.