After the No. 6 typhoon “In-Fa” in 2021 decayed to a tropical depression on July 28, the spiral rainbands around the typhoon caused a sudden increase in precipitation in central Jiangsu Province. Based on the conventional observation data, numerical forecast products and ERA5 reanalysis data, this paper analyzes the process of heavy rainfall. The results show that the ECMWF-FINE numerical forecast product was more accurate for forecasting the situation field. However, the mesoscale CMA models could reflect the magnitude and the extreme precipitation area. Analysis of the reasons for the intensification of precipitation shows that the movement of “In-Fa” was little affected by the guiding air from high altitude; the weak cold air carried by the westerly wind trough reduced the atmospheric stability and forced the warm and humid air to the east of the typhoon to lift when it moved around in Anhui Province. The high and low spatial divergence fields and vorticity fields were concentrated in a relatively narrow area, which constituted an advantageous configuration of vertical mutual coupling. It was conducive to the formation of strong and deep vertical motion, which was an important dynamic mechanism for the generation and persistence of heavy rainfall. The heavy precipitation area was in the warm and humid air flow area of the negative centre of the wet potential vortex (MPV). The negative area of MPV had indicative significance for the direction of the heavy precipitation zone. The distribution of MPV1 and MPV2 reflected the characteristics of convective precipitation caused by the invasion of cold air. In addition, the central part of Jiangsu was located in the highhumidity and highenergy region to the right of the spiral cloud belt. The northeast and southwest airflow converged to form strong linear convective cloud clusters, along with the mesoscale system, which triggered and maintained with the ground convergence line, resulting in extremely heavy precipitation.