An unpredicted heavy rainfall event with a maximum 8-hr accumulated rainfall exceeding 180 mm and a maximum hourly rate of 69.3 mm h-1 occurred in Mianning on June 26, 2020, inducing mountain torrents (MTs). Here, this event is studied using various observational data. The results indicated that (1) the favorable upper and lower circulation and terrain leaded to the rainstorm event. During the rainstorm, the situation at 500 hPa had little change. With the tiny westward west-Pacific subtropical high (WPSH) and the eastward mid-latitude tough, the sourthwestly wind accelerated. The forming and developing of basin vortex (BV) can be explained well by the aspect change of ageostrophic wind. The convergence between the northly wind of western BV and the sourthwestly wind in Panxi Plateau determined the occurrence and development of rainstorm. (2) The rainstorm process can be divided into two stages: The first stage, the south branch of the updraft reached to the upper troposphere by the orographic lift, cold poo lift and convergence at the mid-level troposphere. The second stage, the south branch flow was uplifted by the orographic and the cold air associated with Sichuan basin. The maximum reflectivity over 50 dBZ closed the melting level on account of the maximum vertical velocity location descended during the period of large rainfall intensity in LLS. The maximum VIL over 50 kg/m2 means the high?precipitation efficiency and rain intensity.