The autumn rainfall over Southwest China is important for its agricultural and social effects. Studying the mechanism of its formation helps to improve its future prediction. A set of experiments, including eight stages of mountain uplift, is used to analyze the role of mountain uplift in the formation and development of autumn rainfall over Southwest China. The results show that the mountain uplift, especially the Tibetan Plateau is an essential factor in the formation and development of the autumn rainfall over Southwest China. The amount of the rainfall increases gradually with the mountain uplift. Being the transition season from summer to winter, the rainfall in autumn is influenced by the mountain uplift through dynamic and thermodynamic mechanisms in a different way from that in summer and/or in winter. Dynamically, with the Tibetan Plateau uplift, the upstream westerly is split into two branches, and the southern branch gets enhanced gradually and brings more water vapor from Arabian Sea and Bay of Bengal into the Southwest China region (the summer mode). Meanwhile, the East Asian trough is also strengthened, with the strengthened meridional circulation over mid-high latitudes, which favors the cold air from the north flowing into Southwest China (the winter mode) to converge with the warm humid air from the south (summer mode), resulting in the phenomenon of autumn rainfall over Southwest China. From the thermodynamic perspective, the surface heat source increases over the western part while decreases over the east part with Tibetan Plateau uplift, which promotes the ascending air flow over this area, and plays positive feedback on the increased autumn rainfall.