Abstract: The effects of land-use change on the watershed flood process have become one of the major hydrological concerns in the world. A change in the flood process leads to the alteration in flood design and control system, which has caused serious problems to human life and urban security. To investigate the impact of urbanization on the rainstorm flood, the Shuangqiaobang community in Changzhou City was chosen as the typical highly-urbanized region. An improved method to divide runoff plots was proposed by considering the combination of rivers, roads and drainage sumps to establish a suitable HEC-HMS（Hydrologic Engineering Center-Hydrologic Modeling System）model for a highly-urbanized region. Based on this, the rainstorm flood processes under different return periods were simulated by the different land use situations. The results show that the increase of rainstorm flood would be bigger and more rapid under the transformation process from grassland to urbanization than that from land to urbanization, and the increase rate of peak discharge is less than that of flood volume. With the degree of transformation from other land use patterns to urbanization increase, the differences among the impacts of the different land use patterns on the rainstorm flood increase, especially the impact on the rainstorm flood under low return periods. The results of this study can provide scientific supports for the flood control and water resource management.