The surface of the water will face the problem of river flood when a foundation pit is excavated on the rock foundation steep slope of river. It is necessary to fill the cofferdam outside the foundation pit. The cofferdam project on the steep slope of the rock foundation is not only difficult to meet its own requirements due to its poor stability, but also needs to be evaluated for the impact on the river flooding and navigation. Taking Nanning 324 National Highway Reconstruction and Expansion Project (Jinling Bridge) as an example, the difficulties in the design of rock foundation steep slope cofferdam are introduced. The design scheme is proposed in allusion to the constraints, and its safety and stability are verified, which provide a reference for similar projects.