In this study, ammonia nitrogen (NH3-N), total phosphorus (TP), and the permanganate index (CODMn) were chosen as clustering reference factors. The water quality of the Erjiangsi section of the Jiangan River was evaluated via the gray clustering method. The results indicated that the water quality of the section was classified as Class I from January to March and as Class II from April to December. Pearson correlation analysis revealed minimal impacts of meteorological factors such as temperature, air pressure, and humidity on water quality. An analysis of the monthly data revealed that rainfall and nonpoint source pollution were the primary factors affecting water quality, with nonpoint source pollution having a greater impact than rainfall. The gray clustering method outperformed the single-factor evaluation methods by considering multiple interacting factors, providing a more accurate assessment. The fuzzy comprehensive evaluation also supported the validity and reliability of the gray clustering method for complex water quality data.