At present, the construction load value suffered by the single wall steel cofferdam is usually calculated by the linear load mode based on the current specifications, which cannot truly reflect the actual deformation and stress of the cofferdam and pile cap as a spatial structure during the construction period caused by the hydraulic characteristics. A single wall steel cofferdam in Qingjiang River Bridge was taken as a background project. By considering the change of cofferdam structural form during the construction process comprehensively, a new method with improved load model was established, which are combined by the fluid dynamics and finite element analysis. At the same time, compared with the hydrodynamic calculation method of the current specification, the reliability the structural safety of the new method were evaluated under the complex hydraulic conditions. It is shown that, within the research scope, in terms of concrete force, the existing code calculation results are biased to be unsafe, and their results are biased to be about 20% smaller. While for steel members, the displacement results are closer, and the maximum stress value of the annular secondary ribs calculated by the code is biased to be about 20% smaller. The use of large eddy simulation(LES) turbulence modeling to simulate the complex river flow conditions better reflects the effects of spatially complex water flow forces on the structure. In summary, the research results can be used to ensure the construction safety of the cofferdam structure under complex hydraulic conditions, and can provide a reference basis for the design and calculation of similar single-wall steel cofferdams in the future.