Abstract: With the opening of Gaoji Crossway and the dredging performance in Tongan Bay and in the western waters of Xiamen, the boundary conditions and hydrodynamic environment of Xiamen sea have changed greatly. Based on the latest boundary conditions, topography conditions and hydrology and sediment conditions, the influence of Maluan Bay opening on Tongan Bay and the western waters of Xiamen is studied used mathematical modelling. As results, under the conditions of spring tide when Gaoji Crossway was opened, the net tidal volume generated from the Gaoji Crossway opening was 3 930×10⁸m ³, and the exchange capacity of water in the western waters was significantly enhanced. After further opening of Maluan bay, the total tidal volume will increase in average of 4 560×10⁸ m³ flowing in through Gulangyu sections and Wutong sections to Tongan Bay and the western waters, respectively, with allocation proportion close to 1∶1. The water exchange capacity between Tongan Bay and the western waters can be enhanced after the opening. However, the water quality of Tongan Bay and the western waters will depend on the water quality of Maluan Bay.