Abstract: With design capacity of 170×10⁴　m³/d,the new structure of A/O biological nitrogen removal technology will be applied for the upgrading and rebuilding engineering in Shanghai Zhuyuan No.1 Wastewater Treatment Plant in the long term, while the ordinary aeration technology will be applied inthe near future. Under dry/wet seasons, considering three conditions including influent COD_Cr untreated, influent COD_Cr treated with current chemical-biological flocculation, and treated with upgraded and rebuilt process, mathematical models were applied to simulate and project the intrusion of sewage plant effluent into COD_Cr mixed zone and sensitive area at Changjiang Estuary. The resultsindicate that, under the worst hydrological conditions of dry season, the COD_Cr 100～150 times dilution area is far smaller after upgrading and rebuilding engineering, and the mass concentration of COD_Cr increment (0.28 mg/L) at upstream 8 km approaches to the background value and is also far lower than the other two conditions, and thus will not exert majorimpacts on the water intake at Lingqiao.