[Objective] Analyzing water quality of the Yellow River reservoirs in order to provide a method that is reasonable and effective in reservoir water quality evaluation, and conducive to the comparison of water quality and generalization of it.[Methods] An improved integrated water quality index(WPSPAQI) method was applied to evaluate the water quality in Donghai and Xihai Reservoirs of Binzhou City, Shandong Province based on the water quality data of four monitoring sections from April, 2011 to April, 2014. Firstly, the water pollution index(WPI) value was used to choose proper sub-index. Secondly, the exceeding standard multiple method, principal component analysis and analytic hierarchy process method were used to determine the weight values. Thirdly, the water quality index(WQI) was introduced and the improved method WPSPAQI was used to analyze the overall state of water pollution, of which include the temporal and spatial characteristics.[Results] ① Most of the water body met the standards of class Ⅰ-Ⅱ and the general water quality in the given period was good. ② The proportion of water quality in class Ⅰ gradually increased, but some pollution factors had not been correspondingly improved. In spatial variation, the difference of water quality between the two reservoirs was small. ③ It also showed that the reservoirs have a certain self-purification ability, since the water quality of the inlet was better than that of the outlet.[Conclusion] Compared with other assessment methods of the reservoir water quality, the improved WPSPAQI method is more reliable in the aspect of data processing and weight determination.