Based on multisource observations, reanalysis, and convection-resolving model forecast data, the characteristics and formation mechanism of the sudden rainstorm process on 26 June 2020 in Mianning, Sichuan Province, were analyzed by employing physical quantity diagnosis, standardized anomaly analysis, and comparison with similar processes. The results were as follows: (1) The process was a local sudden rainstorm characterized by several banded meso-γ convective systems and extreme hourly precipitation generated by the “train effect”. The convective cloud clusters had the features of a mesoscale convective complex with a low center of convective echoes. (2) Surface convergence and uplift, formed by the outflow of a convective cold pool in the northern part of Mianning and a strong southerly wind in the valley, triggered convection. (3) The southerly low-level flow in Southwest Sichuan exhibited phased enhancement and provided continuous warm and moist air transportation. Its interaction with the downhill cold pool early in the process and the confluence with the southward cold air from the western basin later in the process repeatedly triggered convective cells on the west and south sides of Mianning station, causing the “train effect” in downstream areas. (4) Physical quantities, such as the convective effective potential energy of the environmental atmosphere, had more significant anomalies and persistence of anomalies compared with a similar process in the past. (5) The high-altitude terrain in the northern part of Southwest Sichuan has a substantial effect on delaying the entry of cold air into the Anning River valley and maintaining unstable stratification in the valley. The forcing uplift of the terrain in this area formed potential convection-triggering conditions in the river valley upstream. Finally, the conceptual model of the formation mechanism of this rainstorm process was presented.