Abstract: Compared with the common rainstorms, abrupt rainstorms are still the difficult and weak point in weather forecasting and research. From 19:00 on June 26 to 02:00 on June 27, 2020, an abrupt rainstorm occurred in Mianning, Liangshan of Sichuan, and caused serious disasters. To further investigate the cause of this rainstorm, the data from observation experiment, satellite remote sensing and ERA5 reanalysis were used to analyze the humid-thermal environment and dynamic characteristics of the abrupt rainstorm process. Results showed that: (1) The rainstorm was caused by a rapidly strengthened MCS (Mesoscale convective system), and establishment of the humid-thermal environment that induced the MCS had a "sudden" feature, that was within 6 to 12 hours before the occurrence of the rainstorm, the CAPE (convective available potential energy) increased rapidly, the amount of precipitable water continuously increased, the humidity structure of “the dry upper level—the wet lower level” and thermally unstable state could be established. (2) A strongly ascending movement was formed during the cyclonic vorticity development and enhancement from mid-upper level to the low level, and the development of low-level cyclonic vorticity was also accompanied by a strong ascending motion, the establishment and enhancement of dynamic conditions which was related to regional topography, played an important role for the maintenance of strong convection and the occurrence of the abrupt rainstorm. (3) The rainstorm process just occurred in the best period of the coupling between low-level helicity and water vapor, and the enhancement of the coupling between vertical helicity and water vapor make it easier to trigger the rainstorm process. The dynamic-water vapor coupling has an important stimulating effect on the rainstorm, and the moist helicity is more indicative for rainstorm area.