Steel sheet pile cofferdam is widely used in sandy soil, viscous soil and deep foundation pit because of its advantages of low cost, strong overall stiffness, short construction period, easy penetration into hard soil and good waterproof performance. Due to complex working conditions and many uncontrollable factors in the construction process, it is necessary to simulate and analyze the construction conditions in advance. The steel cofferdam in the soft soil area is a weathering layer, which is difficult to construct. In order to ensure the structural safety of steel sheet pile cofferdam and control its water use area, the influence of multiple design parameters on the deformation of cofferdam was studied by single factor sensitivity analysis method, and three-dimensional finite element model was calculated by Midas software. The finite element numerical simulation method is used to simulate and calculate each construction condition, analyze the deformation and stress changes of cofferdam and surrounding soil, and the deformation and internal force conditions of steel sheet pile cofferdam and internal support structure in the construction process. Through comprehensive analysis of theory and on-site construction conditions, the construction factors affecting steel sheet pile cofferdam, internal support structure and surrounding soil are obtained. The change of soil gravity stress caused by precipitation during construction leads to excessive deformation and stress concentration. And the establishment of internal support has played a good deformation control phenomenon.