Abstract: The Hedong region of Gansu Province experienced the strongest regional rainstorm rrom 22 to 24 July 2024, since 1961. Based on observations from automatic weather stations, radiosondes, dual-polarization radar, merged precipitation products, and ERA5 reanalysis data, this study preliminarily investigates the precipitation characteristics, evolution of mesoscale convective systems (MCS), synoptic conditions, and mesoscale mechanisms of localized extreme rainstorm. The results are as follows. (1) The event featured extensive heavy rainfall, prolonged duration, and record-breaking accumulations, exhibiting remarkable extremity. The MCS displayed backward propagation and a "train effect," with a long lifespan. Radar echoes indicated warm-rain processes characterized by low centroid heights and high precipitation efficiency. (2) Under the combined influence of the strong and Stable Estern Acific Ubtropical High and Typhoon "Prapiroon", an intense and persistent moisture transport channel extended from the South China Sea to the southeastern Gansu rainstorm area. This led to extreme positive anomalies in precipitable water (standardized anomalies exceeding 4σ), serving as the primary factor. Additionally, anomalous intensification of a mesoscale low vortex in the lower troposphere and the South Asian High in the upper troposphere created favorable conditions for sustained upward motion. (3) The Longshan Mountains induced distinct moisture transport pathways and convective triggering mechanisms at the two extreme rainfall centers (Dingxi Tongwei and Pingliang Chongxin) on its western and eastern flanks. The Tongwei rainstorm resulted from intense moisture convergence and uplift driven by the combined effects of a horn-shaped topography and a lower-tropospheric vortex, with moisture convergence concentrated at 800 hPa and limited vertical development of convection. In contrast, the Chongxin rainstorm was maintained by deep convective systems triggered by wind shear on the left side of a low-level jet (LLJ), pulsations of the LLJ axis, and dynamic coupling between the LLJ and upper-level systems after the LLJ traversed the Longshan Mountains, with moisture convergence focused at 700 hPa. Moderate CAPE and convective instability provided favorable conditions for the prolonged maintenance of MCSs at both rainfall centers.