Abstract: An heavy rainfall event occurred in Mianning on 26 June 2020, inducing mountain torrents (MTs). Here, using various observational data and ERA5 reanalysis data, the causes for the event is studied from aspects of atmospheric circulation, moister conditions, local stratified characters, evolution of convective systems, and orographic effect. The results indicated that: (1) The 500-hPa situation changed little during the rainstorm. The southwesterly wind accelerated due to the tiny westward West Pacific subtropical high and the eastward mid-latitude tough and affected Sichuan Province. The forming and developing of a basin vortex (BV) can be explained well by the clockwise aspect change of ageostrophic wind at the northern part of BV. The convergence of the western BV’s northerly wind and the Panxi Plateau’s southwesterly wind changed the local dynamic and moister condition of rainfall region and determined the occurrence and development of the rainstorm. (2) The rainstorm process can be divided into two stages: In the first stage, the southwesterly wind can break the convective inhibition via the orographic lift, cold pool lift, and convergence at the mid-level troposphere. The deep convection formed on the southwest side and moved to the LSS (Lingshansi station) with heavy rain. In the second stage, the southwesterly flow was uplifted higher by the orographic and cold air associated with the Sichuan basin than the first stage. The strong convective cells occurred incessantly near LSS with the maximum reflectivity of ＞50 dBZ closed the melting level and caused high rainfall intensity.