Problems such as waterlogging and poor flood discharge capacity in urban rivers make the reinforcement of urban river embankments gradually put on the schedule of water conservancy work. Therefore, this paper mainly introduces the foreign free software HEC-RAS and its calculation principle. Taking the county section of Mangpian River’s Comprehensive Treatment Project in Lincang, Yunnan Province as an example, we carry out the simulation research and application of the river water surface line, and compare the simulation calculation results with the design calculation values. The two results agree well, which indicates that HEC-RAS has strong operation ability. It is expected that HEC-RAS will be further studied in the future. The research results can better serve the engineering practice.