Abstract: Supposing the "basic concentration" of each pollutant and substituting the relative value for the monitoring value of pollutant concentration,the parameters in the formula of pollution harm rate of water quality are independent of the characters of pollutants.The formula of pollution harm rate of water quality assessment suited to multipollutant and the formula of comprehensive index of pollution harm are derived by using the genetic algorithm to optimize the parameters.The comprehensive pollution harm index of water quality is calculated by weightting to partial index of pollution harm using the general fuzzy contrast method. The comprehensive assessment method of water quality shows the clearness of physical meaning,easy to calculate,simple to use and the generality and comparison.