Abstract: Based on observations, ERA5 reanalysis data and the data from the Global Data Assimilation System, this study conducts a comparative analysis on the causes of two distinct stages of extreme rainstorm in Hunan Province, China caused by Typhoon Gaemi (No. 2403). The results show that this event occurred under the background of Typhoon Gaemi remnant. The northward-moving residual vortex of Typhoon Gaemi was blocked by a high-pressure dam, and veered westward and southward into Hunan. The continuous water vapor transport on the eastern side of the typhoon circulation contributed to the prolonged maintenance of the residual vortex, thereby resulting in a wide coverage of heavy rainfall and record-breaking accumulated precipitation in many regions.During the first stage, the rainstorm area was concentrated near the typhoon trough within the third quadrant of Typhoon Gaemi, while in the second stage it was centered near the core of the residual vortex. The rainstorms in the two stages exhibited distinct differences in the primary dynamic, thermodynamic and moisture conditions. Specifically, in the first stage, the dominant factors were the typhoon trough, a boundary layer northerly jet is orthogonal to the Nanling Mountains, which significantly enhances the updraft of the windward slope and increases the instability of the atmospheric convection, which directly leads to the extreme rainstorm in Zixing, southern Hunan. In the second stage, after the " Gemei " was stopped, the development of precipitation in western Hunan was affected by the residual vortex shear. The local frontogenesis and the southerly jet of the boundary layer further promoted the enhancement of the inclined ascending motion, and a large number of conditional symmetric unstable energy was released, which eventually formed the extreme short-term over Xupu in western Hunan during the second stage. In this process, the boundary layer jet plays a key role in the development of heavy precipitation. The residual vortex strengthens, and the potential gradient between the residual vortex and the subtropical high increases, which promotes the conversion of baroclinic potential energy to kinetic energy, and the kinetic energy of the precipitation area increases. The growth of kinetic energy promotes the enhancement of the jet stream, which leads to the increase of precipitation. The water vapor in both stages is mainly derived from the low-level jet stream and the boundary layer jet stream, moisture originated from the lower troposphere and was transported to the mid-to-upper levels via ascending motion. However, the vertical transport extended to higher altitudes during the second stage, suggesting a stronger influence of baroclinicity on vertical moisture transport during that period. Furthermore, the characteristics of water vapor transport and budget differed substantially between the two stages. In the first stage, water vapor originating from low-latitude Pacific oceans was transported into the rainstorm area through Fujian, following a north-to-south path along the outer cloud systems of the typhoon. Net moisture influx occurred along the northern and western boundaries. In the second stage, the water vapor transported into the rainstorm area through Indian oceans to the north through South China and transported to the rainstorm area. The south and west of the water vapor were the inflow boundary, which was consistent with the direction of the low-level jet and the boundary layer jet.