Abstract: The water quality of the sea near Zhoushan Islands is significantly lowered in recent years, and the main pollutants are inorganic nitrogen and active phosphate. Numerical simulations have been conducted in this work to examine the water quality of this sea region, focusing on the possible impacts by implementing a planned development in a new district of Zhoushan. The results show that external pollution sources are the major cause of the existing pollution in the sea region. The water quality will be considerably improved by implementing the planned development in the future if the local emission in the Zhoushan district is reduced as planned. However, measures of pollution control, such as classification of pollutants and discharge management, should still be strengthened.