Lake Baihua is an important source of urban drinking water in Guiyang City, and water quality anomalies often occur for many years. The long-term water quality monitoring data (from 2009 to 2018), together with the comprehensive trophic level index(TLI) method and Pearson correlation analysis, are used to study the water quality change characteristics and influencing factors of Lake Baihua over the past ten years. The results show that:1) The concentrations of chlorophyll-a(Chl.a), total phosphorus(TP), total nitrogen(TN), permanganate(COD_Mn) and transparency(SD) ranged from 3.43 to 39.72 mg/m³, from 0.034 to 0.115 mg/L, from 1.200 to 2.759 mg/L, from 1.41 to 5.51 mg/L and from 0.75 to 2.07 m, respectively. The high N/P ratio (12-63) indicates that Lake Baihua is a phosphorus limited type. 2) In space, the concentrations of TP, TN, ammonia nitrogen, COD_Mn and Chl.a decreased gradually along the flow direction of the water body, and SD showed the opposite trend. 3) The water quality of Lake Baihua has gradually improved from class Ⅳ to class Ⅲ from 2009 to 2018, and TLI has changed from light-eutrophic state to mesotrophic state. 4) The inflow tributaries are the main factors affecting the water quality in the reservoir area. The water TP and TN in Dongmenqiao and Nanmen Rivers were seriously over standard, which threatens the stability of water quality in the reservoir area. 5) The concentration of Chl.a was significantly related to temperature, water level, wind speed, and total phosphorus, which is considered to be controlled by the combination of hydrological, meteorological and nutrient factors. In the future, the pollution control of Dongmenqiao, Nanmen Rivers needs to be strengthened. Meanwhile, the mechanism of water quality (algal bloom) influenced by hydrodynamics, climate change and other hydrometeorological factors also needs to be further studied.