In a case study of Yunlong Lake in Xuzhou City in 2015, 180 samples at 15 sampling sites were divided into 13 groups through hierarchical cluster analysis. Using the sample mean of each group as the input, the comprehensive water quality identification index of each group was calculated. The evaluation result of each group was then applied to each original sample for water quality assessment of samples in large amounts, over long periods, and with multiple sections. Based on the results of water quality assessment, the correlation between monthly increase of the NH₄⁺-N and COD_Mn concentrations and precipitation at each sampling site was analyzed. The results show that the water quality of the sampling sites was at class I to class Ⅳ levels, but mostly above the class Ⅲ level. The water quality of the central lake was superior to that of the supplemental water, and the water quality in the dry season was superior to that in the wet season. Correlation analysis showed that the monthly increase of NH₄⁺-N and COD_Mn concentrations had a significantly positive correlation with precipitation. The pollutants brought by precipitation into the lake were one of the main reasons of water quality deterioration in Yunlong Lake.