HEC-RAS is a one-dimensional hydrodynamic model. Constant flow model in HEC-RAS model can be used for analysis and calculation of Shangyoujiang bridge backwater during construction period or upon completion.The results showed that the maximum backwater height value is 0.02min the event of a 5 year flood during construction period of Shangyoujiang bridge. The maximum backwater height value is 0.01m, 0.02m, 0.03m in the event of 5 year,10 year, 50 yearflood aftercompletion of Shangyoujiang bridge.Due to the construction of the Shangyoujiang bridge backwater is very small .its impact on river flood safety is smaller, and less impact on the upper reaches of river drainage. By incorporating the energy balance equation, constant flow model in HEC-RASmodel canbe usedin analysis of pier backwater calculation and influence. The parameter setting is simple, the section layout is convenient, and it is worth popularizing.