From 2008 to 2020, we measured chlorophyll-a (Chl.a) and major environmental factors at five monitoring sites in Xiaojiang River, Three Gorges Reservoir, on a quarterly basis, analysed the spatial and temporal variation characteristics of Chl.a, explored the correlation between Chl.a and environmental factors in water bodies, and analysed the major environmental factors affecting the change of Chl.a in Xiaojiang River. The results showed that the annual average concentration of Chl.a was (20.69±9.41) μg/L with a variation of 5.96-33.90 μg/L. The Chl.a concentration varied seasonally, and the overall characteristics showed that the Chl.a concentration was significantly higher in spring, summer and autumn than in winter, and peaked at 34.48 μg/L in spring. The spatial distribution of Chl.a concentration showed an inverted “N” type distribution pattern, with the highest in the Huangshi section (29.5 μg/L) and the lowest in the Yanglu section (15.23 μg/L). Pearson correlation analysis showed that Chl.a concentration in Xiaojiang was significantly and positively correlated with COD_Mn, conductivity, pH and DO, and significantly and negatively correlated with NO^-₃-N, PO^3-₄-P and Secchi depth. Stepwise regression analysis was used to screen the five most important environmental factors affecting Chl.a. Temporally, the most important environmental factors affecting Chl.a in different seasons were turbidity in spring, DO in summer, NH₃-N in autumn, water temperature in winter. Spatially, the most important factors influencing Chl.a from upstream to downstream in different reaches were conductivity in Qukou, COD_Mn in Yanglu, PO^3-₄-P in Goyang, conductivity in Huangshi, DO in Shuangjiang. DO and COD_Mn were the most important environmental factors affecting Chl.a in Xiaojiang River, where DO had the largest direct effect on Chl.a concentration with a direct path coefficient of 0.451, and COD_Mn had the largest indirect effect on Chl.a concentration with an indirect path coefficient of 0.204. Fitting analysis showed that Chl.a and DO, Chl.a and COD_Mn were best fitted with an exponential function, followed by a power function and finally a linear function. This study is important for the in-depth understanding of the spatial and temporal variation patterns and the main influencing factors of chlorophyll-a, and for the management of eutrophication in Xiaojiang River and other tributaries in the Three Gorges Reservoir area.