Xikeng Reservoir is an important drinking water source in Shenzhen, it is necessary to grasp its water quality status and clarify its pollution sources. In this paper, eight typical indexes, including pH, permanganate index, ammonia nitrogen, nitrate nitrogen, total nitrogen, total phosphorus, iron and manganese, were selected from the water body of Xikeng Reservoir over the time span of January to December, 2019. Based on the analysis of the variations of the indexes and interactions between them, the water quality status was evaluated by the water quality composite index (WQI) method, and the key pollution sources were identified using factor analysis and absolute principal component score combined with multivariate linear regression, then the contribution rates of major pollution sources were estimated. The results showed that under the influence of phytoplankton, pH reached 9.8, permanganate index was 6.3 mg/L in June, ammonia nitrogen concentration was less than 0.1 mg/L annually, nitrate nitrogen accounted for about 71% of total nitrogen, total nitrogen concentration was greater than 0.5 mg/L throughout the year, and the average total phosphorus concentration was 0.014 mg/L; the water body presented phosphorus limitation; the concentrations of Fe and Mn with strong correlation were less than 0.2 mg/L throughout the year. The WQI and Mann-Kendall trend test showed that under the action of various measures, the average annual water quality index of the reservoir was 78 and showed a promising trend. The analysis of pollution sources indicates that the water quality of the reservoir is directly affected by phytoplankton, and the key to solving the problem of reservoir pollution is to control the water diversion and endogenous release of pollutants.