Taking the section of Songhua River in Harbin for example in this paper, taking COD as evaluation factors, and using the economic utility model and AHP, the water environmental capacity value has been studied. The results show that, due to water capacity, the saved cost of wastewater treatment is 0.067 billion yuan/year, and the economic benefits is 9.417 billion yuan/year. The total value of the environmental capacity is 9.484 billion yuan/year. The economic benefits of the class II, III and IV are 3.3,4.802 and 11.32 billion yuan respectively. The maximum value of the environmental capacity belongs to the Class III water body, and is about 51％ of the total economic value.