Abstract:A tidal flow and pollutant transport mathematical model was first established and verified by the measured data. Then, phosphate concentrations under six hydrological conditions (spring, middle and neap tides in the flood season; and spring, middle and neap tides in the dry season) were computed by this verified mathematical 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 allowed discharge amount and cut rate of phosphate at outfalls were calculated. The research results show that the phosphate concentrations at the mixing zone boundaries of Douhe and Shahe rivers exceed their allowed standard values in the dry season, and the exceeding concentration occur in Shuanglonghe mixing zone and reclamation area in the flood season. The phosphate discharge amount from Douhe and Shahe rivers in the dry season and Shuanglonghe in the flood season must be cut down.