Using observation and numerical simulation results, we reveal that three vortexes, namely the northern plateau vortexes (TPV1), southern plateau vortex (TPV2), and Southwest vortex (SWV), developed successively during a disaster-causing rainstorm event in Southwest China from August 5 to 6, 2019, which led to the intensification and eastward propagation of the rainstorm. Through numerical experiments, we study the effects of multi-scale topographic factors (Tibetan Plateau TP, Hengduan Cordillera HC, and Sichuan Basin SB) on vortex evolution. The results show that HC plays a key role in SWV formation, while SB influences the SWV location and intensity. The topography of the SB only affects the intensity of TPV2 but does not change the propagation path. In the absence of HC, the plateau vortex does not propagate. The influence of slope change of the steep terrain at the boundary between TP and SB on vortex development was further analyzed. The steeper the slope, the faster the propagation speed of the plateau vortex, and the stronger the SWV after the merging of TPV2 and SWV. Finally, the impact of the terrain slope on the evolution of vortex intensity was analyzed according to the theory of slantwise vorticity development. As the slope becomes steeper, the development coefficient of inclined vorticity decreases rapidly along the vortex slide path, and the forcing effect on the local tendency of vertical vorticity intensifies the rapid strengthening of vorticity.