Abstract: The appropriative steel cofferdam for slope restoration of a large aqueduct is a kind of underwater steel structure, moving externally and statoc internally, and its structural stability is fundamental under working conditions. According to the design structure condition of the steel cofferdam and considering its dynamic and static loads in water, the mechanical analysis of fluid-structure coupling (FSI) system is made. Firstly, the load characteristics of the steel cofferdam are analyzed, and the tanglesome load condition of the flow field around the steel cofferdam is simplified to the hydrodynamic pressure acting on the structure. Secondly, a finite element model of cofferdam structure is constructed. Numerical simulation software ANSYS is used to analyze the stress state of each component and the characteristics of the whole structure. Studies show that the axial stress of the upstream bracing piece is larger than the Euler critical stress, and the deformation of the guide groove and the faceplate is quite large, which requires structural adjustment, while the stability of the middle and downstream bracing piece meets the requirements. By optimizing and adjusting the structure, the method of ameliorating the structure is put forward, and is successfully applied in engineering practice, thus providing a theoretical support for ameliorating the design of the steel cofferdam.