The heavy metal is not easily degradable, the heavy metal pollution of water is the most harmful pollution for the environment, so, to evaluate the pollution grade is very important for pollution controlling. The fuzzy evaluation method is the most important way to evaluate the degree of pollution today. But in some case, one key factor of heavy metal will decide the pollution level, but the classical fuzzy evaluation method cannot arrive to a correct result, and mislead the subsequent processing of pollution controlling. In the paper, the classical fuzzy evaluation method is analyzed firstly, to improve the classical fuzzy evaluation method by changing the weighting of the factors in order to highlight the key factors is developed then. Case studies comparing the results of the two methods, the result shows that the method proposed in this paper is more reasonable to the problem.