Abstract: Screen the factors of water quality evaluation through the stepwise discriminant analysis method. By hypothesis-testing analysis of the monitoring water quality factors, lead in the most effective factors and establish the discriminant equation. Then the evaluation results of the water quality of an example showed that the posterior probability of factors are over 90% and have a significant impact on the identification and classification. The stability of the established discriminant function and the reliability of the results of the evaluation were enhanced. The appraisal result of the water quality of Shitoukoumen reservoir is Ⅱ from year 2001 to 2004,and it is Ⅲ in 2005. Accord with the fact pollution has shown increasing trend year by year，in line with the facts. The appraisal result of water quality without the variable screening is Ⅱ in 2005，and the posterior probability is about 52%, the result shows the error rate is high.

Key words: stepwise discriminant analysis, waterquality, evaluation, screening, factor