Based on the total primary productivity and evapotranspiration data of vegetation in Southwest China from 2000 to 2020, this study used methods such as trend analysis, partial correlation analysis, and residual trend analysis to explore the spatiotemporal evolution laws and driving mechanisms of vegetation water use efficiency in Southwest China. The results showed that from 2000 to 2020, the vegetation water use efficiency in Southwest China showed a non-significant downward trend over time, with a decrease rate of 0.0024 g/(m2·mm·a) (in terms of carbon content). The overall spatial distribution pattern is high in the south and low in the north, and the spatial distribution is dominated by the total primary productivity of vegetation. The vegetation water use efficiency is positively correlated with relative humidity, sunshine hours, and temperature, and negatively correlated with precipitation and wind speed. The relative contribution of climate change to changes in vegetation water use efficiency is 52.6%, while the relative contribution of human activities is 47.4%. Climate change and human activities have a negative impact on vegetation water use efficiency in Guangxi hills, the Yunnan Guizhou Plateau and Sichuan Basin, while the positive and negative impacts in Hengduan Mountains and Ruoergai Plateau account for the same proportion.