Abstract: The effect of sewage discharge on sea water quality in Tongzhou bay is calculated and analyzed by the numerical simulation method. Sewage water is assumed to discharge at the water depth of -4 m, -6 m, -8 m and -10 m during whole day、from high water stand to fastest ebb、from high water stand to low water stand while the sewage discharge rate is 100000 m³/d and the concentration of COD is 50 mg/L.The results show that the affected areas are different because of different hydrodynamic conditions.With the increase of water depth, the water exchange capacity is stronger, and the affected areas become smaller; Under the certain amount of sewage, the affected areas are also changed while sewage is discharged in different time period. The effect is minimal when sewage water is discharged during whole day, but discharging sewage from high water stand to fastest ebb and from high water stand to low water stand prevents sewage from polluting coastal waters to a certain extent.