Abstract: Residual currents play a crucial role in the long-term transport and diffusion of pollutants in water bodies and shoreline changes caused by reclamation activities in Bohai Bay can alter the residual current field. To investigate the changes in the residual current field of Bohai Bay before and after reclamation, the hydrodynamic field and the transport trajectories of Lagrangian particle were simulated using the hydrodynamic module and Lagrangian particle tracking model of MIKE21. Based on these simulations, the Eulerian residual current field and detailed circulation structures in Bohai Bay were obtained. By comparing changes in the hydrodynamic field, residual current field, and Lagrangian particle trajectories before and after reclamation, the following conclusions were drawn: After reclamation, the maximum flood and ebb velocities in most areas of Bohai Bay decreased compared to those before reclamation. The directions of maximum flood and ebb currents near the reclamation area changed, and significant alterations in residual current velocities were observed near the shore and ports. The residual current field in Bohai Bay exhibits four circulation structures, among which the clockwise circulation near Huanghua Port and Binzhou Port and the counterclockwise circulation at the southern bay mouth were significantly affected by reclamation activities. The overall circulation structure of Bohai Bay follows a pattern of inflow from the north and outflow from the south. After entering the bay from the northern part of the bay mouth, the current flows inward and diverges into three branches in the southeastern sea area of Caofeidian, moving southward, southeastward, and southwestward, respectively. The southwestward branch further diverges near Tianjin Port during its inward propagation, participating in the circulation structures on both the north and south sides of Tianjin Port.