Abstract:Two persistent extreme rainstorms occurred in Sichuan Basin during 10-13 and 14-18 August 2020 resulting in secondary disasters， casualties， and huge economic losses.To deeply understand the development mechanism of extreme rainstorms and the disaster-causing mechanism, using various observations and ERA5 reanalysis data, we comparatively analyze the precipitation characteristics of these two rainstorms, and the development, evolution and trigger mechanism of mesoscale convective systems (MCSs) in the heaviest precipitation stage. The results show that the two rainstorm processes both occurred under the circulation background with “two troughs and one ridge” in the middle and high latitudes. They were typical rainstorms accompanied by “east-high-pressure and west-low-pressure” in the basin， and brought precipitation over 250 mm·d-1 (or 100 mm·h-1). The hourly rainfall of the 10-13 August rainstorm exceeded the historical extremes，while that of the 14-18 August rainstorm was equivalent to the historical statistical value. The most intense precipitation stage of the 10-13 August rainstorm was a warm-sector rainstorm， which was caused by a mesoscale convective complex occurrence-development-maturation-weakening process. The radar echo areas ≥40 dBz in this rainstorm were wide and long-lasting. And the echo centroid was low and the intensity was more than 55 dBz. The heaviest precipitation phase of the 14-18 August rainstorm was a mixed precipitation induced by a two α-MCS occurrence-development-merger-weakening process. The radar echo areas ≥40 dBz were narrow and short-lived. The echo centroid was low and the intensity reached 50 dBz. Convection in the 10-13 August rainstorm was produced by horn-mouth terrain flow， windward slop uplift and high temperature gradient zone， and was sustained with strong warm advection at low level， weak cold advection at high level at the same time. The 14-18 August rainstorm convection was triggered by the convergence of the lower troposphere cold， warm currents and the left convergence of the low-altitude jet stream in the warm zone. The shear formed by the intersection of the cold and warm currents led to the persistence of the precipitation.