[Objective] The water environment quality of Pingzhai Reservoir was evaluated for the core water source project of the first phase of the Qianzhong water diversion project, in order to provide scientific reference for the evaluation and scientific management of the water environment quality of the reservoir.[Methods] This paper selected seven sections of Pingzhai Reservoir for water sampling in January, May and August 2018. Dissolved oxygen (DO), chemical oxygen demand (COD), total nitrogen (TN) and ammonia nitrogen (NH₃-N) factors were selected as water quality evaluation index. Based on the combination of analytic hierarchy process and entropy weight method, the weight of each evaluation index was determined, and the water quality of Pingzhai Reservoir was evaluated by Bayesian model.[Results] ① From the combination weight, TN and COD showed great impact on water quality of Pingzhai Reservoir, while DO and NH₃-N showed little impact. ② The water quality of Pingzhai Reservoir was the best in high-flow period, followed by the low-flow period, and then followed by the moderate-flow period. Among the sections, the water quality of reservoir area 1 (KQ1), reservoir area 2 (KQ2) and Nayong river sections (NY) were the worst. ③ TN and COD were the main pollution factors in Pingzhai Reservoir.[Conclusion] The water quality evaluation method based on the combined weighted Bayesian model not only distinguishes the difference of contribution of each evaluation factor to water quality, but also evaluates the water quality accurately. Overall, the water quality of Pingzhai Reservoir meets the corresponding requirements of water function zoning in Guizhou Province.