Based on the construction of lock type steel pipe pile cofferdam of bridge pier of a city expressway bridge, the modeling of PLAXIS 3D is used to simulate the construction process of cofferdam step by step, the spatial effect of lateral deformation of cofferdams under hydrostatic pressure alone and considering the combined action of hydrostatic pressure and flowing water pressure is studied. Based on this, the influence of different velocity, diameter of steel pile, embedded depth and retaining structure form on lateral deformation is analyzed. The results show that: Under the combined action of hydrostatic pressure and flowing water pressure, the lateral deformation of the upstream surface of the retaining structure is much larger than that under the consideration of hydrostatic pressure only, and the maximum deformation occurs at the center line; The lateral deformation distribution of the upstream water is consistent with the vertical velocity distribution of the open channel flow, and the maximum deformation point is located at the maximum corresponding depth of the velocity; The lateral deformation of cofferdam can be reduced by increasing the diameter of steel pile. When the steel pile is embedded in moderately weathered sandstone, the depth of embedded steel pile has little influence on the lateral deformation; The circular enclosure can greatly reduce the influence of flow force on the lateral deformation of cofferdam, and the overall load and deformation distribution are more uniform than rectangular.