Abstract: Based on the ERA5 HD hourly reanalysis data as well as the data of high density AWS and aerological observation, the dynamic and thermal conditions during an extreme rainfall process in southwestern Sichuan Basin on August 17, 2020 are analyzed. The results show that: (1) High and low level jet stream, 500hPa upper trough and low level shear line were the dynamic conditions of this rainstorm. (2) The low level cold air intrusion contributed to the occurrence of obvious frontogenesis in the west of Sichuan Basin, and the southwest of the Basin was in a continuous frontogenesis over time; the frontogenesis intensity had certain indication action on rainstorm intensity and rainfall area. (3) The precipitation area was featured by high temperature, high humidity and significant instability characteristics; during the heavy precipitation period, there were troposphere instability and symmetric instability in the lower troposphere, and the evolvement trends of their intensities were closely related to precipitation intensity; the negative perturbation distribution of MPV₁ was consistent with that of MPV, indicating that the convective instable energy played a major role in precipitation during this period.