Abstract: It is of great significance to study the spatial and temporal variation characteristics of evapotranspiration (ET) and explore its driving factors for the scientific allocation and rational utilization of water resources in southwest China. Using MODIS16 ET data, contemporaneous meteorological data and DEM data, based on trend analysis, coefficient of variation method and geographic detector method, the spatial and temporal variation and volatility of ET and the driving factors were analysed in southwest China during 2000-2020. Results showed that: (1) The interannual ET showed an increasing trend with fluctuation in southwest China and each geomorphic regions, especially in Guangxi hilly region. (2) The annual average ET was relatively higher in southwest and southeast, and relatively lower in other regions in southwest China. The interannual ET was dominated by relatively low fluctuation regions. The fluctuation was obviously in Sichuan Basin. The ratio of area where ET showing a significant increasing trend were all greater than 50% in different geomorphic regions, and the ratio in Ruoergai Plateau was the largest. (3) Factor detection results showed that precipitation was the dominant factor that influenced the spatial heterogeneity of ET in southwest China, with an explanatory power of 57.3%. Climate was the dominant factor resulting in the spatial heterogeneity of ET in Yunnan-Guizhou Plateau, Ruoergai Plateau and Hengduan Mountain, while the dominant factor was vegetation in other geomorphic regions. The dual-factor interactive detection results showed nonlinear enhancement or dual-factor enhancement in the study area. The interaction between precipitation and NDVI explained 64.9% of ET’s spatial difference in southwest China. The interaction between climatic factors and elevation in the Ruoergai Plateau, and the interaction between climatic factors and elevation or NDVI in other geomorphic regions were the dominant factors that caused the spatial differentiation of ET. The study revealed the spatio-temporal variation characteristics of evapotranspiration and the corresponding influencing factor, which could provide scientific decisions for the rational use of water resources in southwest China.