To investigate the water quality status of the Xikeng Reservoir, a subtropical water source reservoir, monthly monitoring of the water quality in the reservoir was conducted from January 2016 to December 2020, so as to obtain the changing trends of 13 indictors, including pH value, permanganate index, biochemical oxygen demand for five days, fecal E. coli, ammonia nitrogen, nitrate nitrogen, total nitrogen, total phosphorus, sulfate, chloride, fluoride, iron, and manganese. The water quality index (WQI) was calculated to evaluate the water quality status, and the WQI model was optimized based on multiple linear regression. The results show that the reservoir water quality is generally good, the pH value and permanganate index exceed the standard in some periods due to the influence of phytoplankton reproduction, the mass concentration of total nitrogen is relatively high, the mass concentration of total phosphorus and number of fecal E. coli groups are high in some periods, and the uptrend of iron mass concentration should be paid much attention; it is difficult to fully reflect the impact of stratification on water quality only through surface monitoring, therefore, monitoring along the vertical direction should be used to fully obtain the water quality risk of the reservoir; the WQI model composed of main indictors, including total phosphorus, iron, permanganate index, ammonia nitrogen, and nitrate nitrogen, is an effective tool for water quality assessment and management.