Based on analysis of commonly used water quality evaluation methods, the weighted gray correlation method was proposed in this paper. A case study of Songhua river in Harbin section was carried out by the water monitoring data in 2009, and it showed that, contrasted to the traditional equal weight gray correlation method, the grey weighted correlation method, according to different functional of every section, could determine the evaluation indicator’s weights, strengthen the impact of concentrations of indicators, and consider every indicator’s impact, so that the accuracy of water evaluation results could be improved.