To study on the asymmetric water and soil pressure on both sides of the cofferdam and the influence of seepage force on the bridge cofferdam, the Nanchang Fuxing Bridge cofferdam was taken as the research object. Based on the fluid structure coupling theory, the deformation law of the locking steel pipe piles under the action of biased pressure combination was studied. A three-dimensional numerical analysis model of the entire process of cofferdam dewatering and excavation was established, and the study was conducted from three aspects: the length of the steel pipe piles, the height of the water level outside the cofferdam, and the angle of terrain undulation. The research results indicate that as the terrain undulation angle increases, the maximum deformation of the soil on the biased side gradually decreases, while the maximum deformation of the soil on the non biased side gradually increases. With the increase of water level height, the maximum horizontal displacement of steel pipe piles on the biased and non biased sides increases linearly, and at the same time, the excess pore water pressure inside the dam accelerates with the rise of water level. As the length of the steel pipe pile decreases, the cofferdam as a whole tends to tilt and turn towards a biased direction. The maximum negative bending moment of the steel pipe piles on both sides decreases as the steel pipe piles decrease, and the stress mode of the pile body changes.