With rapid economic and social development and population growth, rail transit project construction increases obviously at flood detention basin near big city, but few reasearchers turn their eyes to the influence of rail transit on usage of flood detention basin. In view of this, we take rail transit No. 21 in Wuhan as an example. We apply dynamic boundary simulation technology and establish a two-dimensional water flow mathematical model for the flood detention basin to analyze the influence on the basin. Results show that effective flood detention volume occupied by bridge pier or railway station is relatively small in Wuhu flood detention basin. Furthermore, project construction has little influence on flood diversion discharge, duration, water level and flow field. It is the same for flood fall discharge,so project construction doesn’t have obvious adverse impact on the usage of flood detention basin. Finally, the research method and results can be taken as reference for design companies and autorities of flood prevention.