Abstract: Based on multi-source observational data, including surface automatic weather station observations, the European Centre for Medium-Range Weather Forecasts fifth-generation reanalysis (ERA5) data, weather radar data, and Variational Doppler Radar Analysis System (VDRAS) data, this study analyzes the basic observational characteristics and mechanisms of an extreme short-duration rainfall event in Tianjin on July 3, 2022, which exceeded historical records. The analysis was conducted from three perspectives: large-scale circulation patterns, the initiation and development of mesoscale convective systems, and cloud microphysical scales. The results indicate that: (1) From the large-scale circulation patterns, the pre-precipitation environment was characterized by high temperature, high humidity, and high energy features. These conditions, combined with a southerly airflow—formed by the confluence of southwesterly warm and moist flow preceding the 500 hPa upper-level trough and southeasterly flow north of Typhoon Chaba, provided favorable moisture and warm, moist environmental conditions for the event. (2) From the meso-scale perspective, the rainfall event was jointly triggered by the cold pool outflow from upstream precipitation, an easterly disturbance within the boundary layer, and a surface convergence line. Meso-scale convergence, and its interaction with the strong pre-trough wind speeds, generated a meridional mesoscale secondary circulation. This circulation further promoted the development and maintenance of vertical motion over the Jinnan area, consequently leading to the initiation and intensification of the heavy rainfall event. (3) From the cloud microphysical scale, the event was characterized by sustained and stable large raindrop diameters and high number concentrations, exhibiting features typical of "continental" convective precipitation. Abrupt changes in the raindrop size distribution (DSD) microphysical parameters indicate that during this heavy rainfall event, intense near-surface moisture convergence occurred over Jinnan, causing atmospheric humidity to reach saturation or even supersaturation.