[Objective] To find out the key polluted area and identify the main pollution factors of Jiaxing City.[Methods] We set up 29 monitoring sections in the main river in Jiaxing City and the surrounding area and indicators of COD_Cr, E.coli, TN, TP and NH₄⁺-N were monitored. These multiple water quality parameters were combined into a comprehensive index using entropy weight method.[Results] TN, TP of Jiaxing City outranged seriously of the quality standards(GB3838-2002) in Ⅴ class standard. The concentrations ranged 2.60~10.52 mg/L, 0.60~5.68 mg/L. Comprehensive index of water quality of the 29 monitoring points ranged as 1.61~8.69. The most serious pollution of the region is at the junction of Haiyan County and Pinghu City, and Central Xiuzhou District. Other districts around the key polluted region had a decreased pollution. Tongxiang, Western Haining and the northern part of Jiashan County is relatively less polluted. TP was identified as the main pollution factors, COD_Cr, E.coli followed.[Conclusion] In more serious polluted areas, water quality index method can better describe the pollution differences among different areas. The contribution of different water quality parameters to the comprehensive index can identify the main pollution factors.