Based on the hydrological, tidal, and geological characteristics of the Nandu River and the characteristics of the Wenming East Road Tunnel project, the key design technologies of section, stage and elevation of cofferdam and depth of tunnel are analyzed. The design schemes of cofferdam and foundation pit are compared in terms of flood control evaluation, construction period and cost. The study shows that: (1) The rational determination of cofferdam layout scheme and construction period is the key to the success of the project. (2) Compared to double-row pile, the structural internal force, deformation and settlement by adopting the large excavation scheme with both sides of the slope are much smaller, and the cofferdam structure is safer, which can greatly shorten the construction period and save the project cost.