Abstract: A two-dimensional hydrodynamic model is established by using MIKE21 FM. Taking Nanpai River Bridge in Huanghua City of Cangzhou City of Hebei Province as an example, the water level and flow pattern before and after the construction of the urban bridge are simulated. At the same time, two methods are used to calculate the backwater effect of the bridge, so as to analyze the impact of bridge construction on flood discharge, river regime, the third party and the safety of the project. The results show that the established two-dimensional model is relatively accurate and the backwater results are reasonable, which can be used as the basis for flood impact analysis of bridge construction. The raising of water level in upstream of the bridge is 0.016~0.018 m after the bridge construction, the average velocity at the bridge pier increases by 0.05 m/s, the elevation of the bridge beam bottom meets the requirements, and the engineering construction has little impact on the river flood capacity, river situation and third party.