The study assessed the water quality of Danjiangkou Reservoir and its tributaries in China using a water quality index. The water quality in the reservoir area and main rivers was found to be excellent, while some tributary rivers were seriously polluted. Parameters like fecal coliform bacteria, CODcr, BOD5, and ammonia nitrogen exceeded safe levels in some areas. Fecal coliform bacteria, TP, ammonia nitrogen, and CODMn were the most common pollutants.