Abstract: Combined with the reconstruction project of Beijing-Hong Kong-Macao Expressway bridge crossing Dianqi River, this paper adopted Mike21 model to conduct two-dimensional numerical simulation of backwater level and flow velocity of the river before and after the bridge reconstruction considering the overflow of the river. According to the calculation results, the change of backwater level and flow rate of the river before and after the bridge rebuilding is analyzed when the standard flood occurs once in 20 years. The water level and overflow of the river under the design flood control standard of the bridge (once in 100 years) are also analyzed. After comprehensive analysis, the reconstruction of the bridge can significantly improve the flood discharge capacity of the river, which is beneficial to the safety of the river itself and the surrounding construction area. It can provide a reliable basis for the construction of this project, and also provide a certain reference for the construction of similar projects.