Abstract: According to conventional observation data, dual-polarization radar data, and ERA5 (0.25°×0.25°) reanalysis data, the weather background, environmental conditions, triggering mechanisms of the mesoscale convective system and radar parameter characteristics of two extreme rainfall events in Zhejiang on July 16 and 22, 2023, are compared and analyzed. The results show the following: (1) Both processes occur under a similar 500-hPa circulation pattern, with a westerly trough moving eastward around the Subtropical High and a typhoon or tropical depression in the South China Sea or the Western Pacific east of the Philippines. Both events are triggered by a surface mesoscale convergence line. (2) The background characteristics of the middle-low levels and precipitation areas differ. Process 1 July 16, 2023, 1800 BJT (Beijing time) to 2400 BJT occurs under a cold shear at 850 hPa. The rainfall center is located in Jiaxing on the edge of the Subtropical High, and the rain belt extends in a north–south direction. In contrast, Process 2 (July 22, 2023, 1400 BJT to 2000 BJT) occurs under a consistent southwest airflow at 850 hPa, with the rainfall center located in Hangzhou on the northwest edge of the Subtropical High and the rain belt extending in an east–west direction. (3) The thermal and dynamic conditions differ. In Process 1, the continuous strengthening of the southeast airflow at 925 hPa enhances the transport of warm, moist advection in the boundary layer of the rainstorm area, and the energy loss caused by heavy rainfall is promptly replenished. Moreover, cold and warm advection increases synchronously and vertically, leading to highly unstable atmospheric stratification. In addition, the continuous strengthening of the southeast airflow causes wind speed convergence, strong horizontal frontogenesis, and vertical frontogenesis in the middle and upper levels. In Process 2, the southwest airflow in the boundary layer is weak, resulting in the weak transport of warm, moist advection and limited layer instability. The energy loss due to heavy precipitation cannot be replenished promptly, and the entire layer is mainly dominated by vertical frontogenesis. (4) The mesoscale convective systems and microphysical characteristics differ. In the early stage of Process 1, the system is formed by several convective cells that merge and move northward, creating β-scale heavy precipitation cloud clusters. In the later stage, the system is driven by the “backward propagation” and “train effect” of mesoscale precipitation clouds. Precipitation particles are mainly small raindrops in this process. In Process 2, the system is caused by the “train effect” of multiple newly formed convective cells, with precipitation mainly consisting of large particles and low concentrations in the early stage. In the later stage, the system results from the combination and strengthening of systematic linear convection and locally generated convective cloud clusters, with precipitation mainly consisting of particles with small diameters and high concentrations.