The Bayes method is based on the normal principles to analyze and evaluate the water quality of the Huangbizhuang Reservoir,the idea and calculation method of the improved Bayes method are also given. By using GB3838-2002 as evaluation criterion,the improved Bayesian model for water quality evaluation is established through selecting water quality monitoring data on dam,Huangbizhuang Reservoir. The evaluation results show that the model is suitable for Huangbizhuang reservoir water quality simulation,the water quality of Huangbizhuang Reservoir is mainly Ⅰ and Ⅱ grade,while it is worse in the rain season for water quality being Ⅱ grade,other months achieve Ⅰ standard.