Abstract: Based on the conventional observation data, the ERA-5 reanalysis data and the hourly cloud top TBB (black body temperature) data of FY-2G, the development of southwest vortex and the triggering mechanism of rainstorm under two different unstable conditions in Sichuan Basin were analyzed by moist potential vortex (MPV) diagnosis. The following results were observed: The two rainstorm processes were related to the maintenance of the long-lived Mesoscale Convective System ( MCS ). The first process was a typical"east high west low" circulation situation, which was affected by cold air. MCS was a frontal cloud cluster, and the convective system was maintained and strengthened in the intersection area of cold and warm air. During the process, the dry and cold air invaded downward along the isentropic direction, MPV<0, ∂θse/∂p<0, the convective area satisfied the conditional symmetric instability, and the inclined convection occurred, which was an important reason for the enhancement of precipitation intensity. The appearance of positive abnormal value of MPV1 and the negative growth of MPV2 reflected the enhancement of conditional symmetric instability, |MPV1|<|MPV2|, which indicated that MPV2 contributed greatly to the development and maintenance of southwest vortex and rainstorm under the conditional symmetric instability stratification. The second process was a warm-sector rainstorm controlled by the subtropical high, with weak baroclinicity. The MCS was a warm cloud cluster, and the convective system developed under the environment of warming and humidifying in the middle and lower layers. During the process, MPV<0, ∂θse/∂p>0, the atmosphere was dominated by convective instability mechanism, with weak baroclinicity. The vertical dipole structure of MPV1, which was "positive at the bottom and negative at the top", was conducive to the increase of vorticity. The deep vertical upward movement and the low level torrent jointly caused the enhancement of the vertical shear of the horizontal wind, which was an important reason for the development of the southwest vortex and rainstorm under the conditions of convective instability. The negative increase of MPV2 was well corresponding to the development of the southwest vortex and the enhancement of precipitation intensity, MPV1 was a poor indicator of precipitation, but it was a good response to atmospheric instability.