Abstract: The cyclic variation in water quality, water quality trend and differences and influencing factors of ZhongshanReservior were analysis based on the water quality monitoring data from 2013—2023. The results showed that the annual water quality generally fell within grades II～III, predominantly Grade II with mesotrophic state. There was no obvious inter-annual variation but with distinguished monthly difference. The water quality were mostly grade II in spring, summer and winter, grade III in fall. The dominant pollutants were in order TP, pH, ammonia, TN, and COD_Mn, among which, TP was drawn attention. It accounted for 73.3% of the number of times exceeding the standard. Using the seasonal Kendall test, the water quality of Zhongshan Reservoir showed an overall upward trend. Key indicators including total phosphorus, ammonia nitrogen, and COD_Mn were increasing, whereas total nitrogen was declining. The main factors influencing water quality of the reservoir included short-duration, high-intensity rainfall–runoff events, variations in reservoir storage, proliferation of planktonic algae, changes in water temperature and evaporation, and the heavy-metal content of the reservoir-bed sediments. In response to these issues, recommendations were proposed for optimizing water-supply operations and implementing source-water protection measures.