Based on multi-source data including data of automatic weather stations in the surrounding regions of Shanghai, operational radiosonde observations, ERA5 reanalysis data and dual-polarization radar data, this study comparatively analyzes the environmental conditions and mesoscale causes of two extreme rainstorm events in Shanghai on 18 July and 8 September 2025. The main conclusions are as follows. (1) At the level of circulation background, both cases are jointly influenced by the continental high, subtropical high, northeast cold vortex and typhoon. Shanghai is located near a saddle-shaped field with a stable circulation pattern and sufficient water vapor supply. The radiosonde-derived atmospheric stratification exhibits convective instability with a deep moist layer, providing favorable environmental conditions for the initiation and prolonged maintenance of extreme rainstorm. (2) At the level of mesoscale system, the urban heat island effect is prominent in Shanghai. The urban-suburban temperature difference induces local thermal circulation, which promotes the formation and development of convergence lines, serving as the convective triggering condition for the rainstorm. (3) Convective cells are initiated near the convergence lines and undergo backward propagation and train effect during their development and evolution, resulting in the slow movement of multi-cell storms and stagnation of heavy rainfall belts. Some of the multi-cell storms intensify into supercell storms, characterized by low centroid height of strong echoes, tilted structure, deep mesoscale vortex, and long-lasting intense ZDR columns and KDP columns. These features indicate the maintenance of strong upward motion, leading to the supercells’ long duration and high precipitation efficiency. Integrating the aforementioned key factors, a conceptual model is summarized and refined, describing the process where convection is triggered by mesoscale systems and evolves into supercell storms under the coupling effect of multiple synoptic-scale systems. This study aims to provide effective references for the forecasting and early warning of such weather events.