Abstract: In the current practice,water-quality monitoring is usually based on analysis of sampling at the instantaneous moment,without measuring water discharges.Thus,the monitoring values obtatined can only represent the instantaneous concentration of pollutant.The less the number of times of monitoring,the less adequate the representative values are.This would result in incorrect water-quality assessments,because they are based on only a few data which cannot be taken as adequate and appropriate.In this paper,the current method is discussed and a new method based on a concept of statistical concentration of water quality is proposed.Some questions on water quality assessment by the method of total quantity control of pollutant are also addresed.The new method was applied to the water quality assessment in the upper reach of Tou River,Sichuan Province.The result was found to be quite reasonable and feasible.