In order to analyze the impact of the underlying surface changes caused by human activities on urban flood risk in plain areas, a hydrological-hydrodynamic model was constructed using MIKE 11 to analyze the changes of the underlying surface of the urban flood control encirclement of Jiaxing City from 1990 to 2020, and the corresponding urban flood process and its change characteristics were simulated using different underlying surface conditions as input conditions. The results show that the urbanization process was rapid from 1990 to 2020, with approximately 46% of the arable land converted to construction land, and the expansion of construction land has increasingly amplified the flood risk as the rainfall return period increases. Under a 50-year return period rainfall event, the highest flood level was 0.206 m higher than a 5-year return period event. Under the underlying surface conditions in 2020, 12.3% of the river channels were unable to meet the 30-year return period drainage standard. It is expected that flood threats will further intensify with continued urban development, and corresponding measures must be taken to enhance urban drainage capacity.