<p>Extreme rainstorms have occurred frequently in the western part of the Sichuan Basin (SCB)—an representative area of the steep terrain on the east side of the Tibetan Plateau—but its prediction remains a considerable challenge. In order to understand the circulation-related causes of extreme rainstorms in the region, we analyzed three representative cases, and the results showed that the circulation pattern that dominated the three extreme rainstorms was one involving “east high–west low” horizontal circulation and double secondary vertical circulations in two dataset of NCEP_FNL and ERA5. The former was mainly composed of troughs over the western plateau of Sichuan and the western Pacific subtropical high (WPSH), while the latter was composed of two closed vertical circulations in the meridional and zonal directions. Influenced by the two circulations, the favourable dynamic coupling conditions between water vapour and weather systems were built over the western part of SCB, which in turn led to violent activities of mesoscale convective systems (MCSs) and precipitation. Under the influence of cyclonic vertical circulation in the meridional direction, MCSs were able to move along the south–north (north–south) direction over the western SCB; while under the influence of anticyclonic vertical circulation in the latitudinal direction, it was difficult for MCSs to develop in the descending airflow east of 107°E. This finding emphasizes the importance of double secondary vertical circulations and can provide a reference for understanding the extreme rainstorms on the east side of the Tibetan Plateau.</p>