Caofeidian is a large harbor being constructed in north of China. It is greatly meaningful to investigate the pollution situation of this area. In this paper, firstly, a tidal flow and pollutant transport numerical model based on Delft 3D-FLOW was established and verified by measured data to simulate and analyze phosphate distribution in Caofeidian sea area. Then, phosphate concentration under six hydrological conditions (spring, middle and neap tides of flood season; and spring, middle and neap tides of dry season) was computed by this numerical model, and the comparisons were made between the computed phosphate concentration at the boundary of the mixing zones and corresponding water quality standards. At last, the allowances discharge amount and cut rate of phosphate of each outfall was calculated. The results show that the high phosphate concentration appears the sea area around the river outlets of Douhe and Shahe, and the phosphate concentration of Douhe and Shuanglonghe rivers’ mixing zones exceed their allowed standard values, the total phosphate discharge from these rivers must be cut down.