To explore the pollutants sources and the water quality variation rules of the water source reservoirs, monthly monitoring of water quality from April 2013 to April 2014 was carried out in Zhelin Reservoir, which is the main water source of Jiujiang City. The dynamic variations of water temperature and DO concentrations were measured in situ, while COD_Mn, TOC, TN, TP, Fe and Mn were determined in the laboratory. The results showed that the main pollution source of Zhelin Reservoir was the upstream runoff, and its percentage contribution to the bulk pollutants was 88.3% for COD_Mn, 76.2% for TN and 67.6% for TP, respectively. During the stable stratification period, the maximum concentrations of COD_Mn, TOC, TN, TP, Fe and Mn in the bottom water reached 3.8, 2.4, 1.12, 0.14, 0.42 and 0.34 mg/L, respectively. The algae population and algal species identification results demonstrated that the maximum density of algae was 2.25×10⁶ cells/L, in which green algae was predominated. However, during the mixing period, Mn concentration exceeded the drinking water standard.