A rainstorm in the west of Sichuan Basin was closely related to the occurrence and development of overshooting convection on August 19, 2019. This paper used the ERA5 reanalysis data and high-resolution numerical simulation results of WRF to reveal the formation mechanism of this overshooting convection. The results indicate that this overshooting convection process occurred under a special background that the large-scale tropopause folding in the coastal areas of China resulted in the downward of stratospheric potential vorticity (PV) over the east of Sichuan Basin. Meanwhile, there was a Tibetan plateau vortex on the Tibet Plateau and a Southwest China vortex in the basin. The overshooting convection process can be divided into three stages. (1) The convective initiation. The continental high extended westward, which led to the enhancement of pressure gradient near Sichuan Basin and then caused the low-level jet (LLJ). The cyclonic shear produced by LLJ made the airflow orthogonal to the southeast of the Tibet Plateau strengthen. In the case of the atmospheric instability, the dynamic uplift of terrain and the convergence uplift of airflow triggered the convection. (2) The development of ascending motion in the upper troposphere. The ascending motion in this case mainly related to the PV air-block developing with the diabatic heating. And the generation of PV air-block is from downward transferring of the stratospheric PV persistently resulting from the turbulent activity. The right side of the positive PV anomaly under the easterly flow is usually upward movement, which led to the development and enhancement of ascending motion from 300 hPa to the bottom of stratosphere. (3) The updraft of upper-middle-lower level near the basin was coupled and superimposed to form overshooting convection. The stratiform pattern of Mesoscale Convective System (MCS) and evaporative cooling of water vapor in dry environment caused the downdraft from 300 hPa to 600 hPa. The atmosphere maintained the ascending motion in the upper troposphere, at the same time, dry intrusion occurred in the middle troposphere, which not only intensified the unstable stratification of " upper-layer dry and lower-layer wet " over the basin, but also enhanced the convergence of air flow in the middle and low troposphere, resulting in the downdraft near 300-600 hPa in the middle troposphere turning into updraft. Due to the slantwise vorticity development in the lower layer, the ascending motion can be maintained. As a result, the ascending motion over Sichuan basin appears vertical superposition and coupling, which shows the consistent ascending motion from the lower troposphere to the lower stratosphere and means the formation of overshooting convection and the enhancement of precipitation.