Abstract: Based on the multi-uncertainty characteristics of river hydrology and water quality information,the blind number theory in unascertained mathematics is used to calculate river water environmental capacity.Through the calculation,not only the expected value of allowable load of a predominant pollutant for a river but also its corresponding confidence level can be obtained.The case study shows that the results received from the unascertained mathematics theory accord closer with practical situation than the traditional method of water environmental capacity calculation.