Abstract: Based on the fact that the overall trend of precipitation in May in Jiangnan has been upward since 1980, this paper takes the precipitation in May in Jiangnan from 1980 to 2021 as the research object, and uses the data of CPC (Climate Prediction Center) precipitation, NCEP (National Centers for Environmental Prediction) atmospheric circulation, and Hadley center sea surface temperature to analyze the precipitation trend characteristics in May in Jiangnan, diagnose the water vapor budget, and finally discuss the possible physical processes. The results indicate that the dynamic component of precipitation in Jiangnan in May is one order of magnitude larger than the thermal component, and both of them have no obvious trend change from 1980 to 2005, but show significant increase trends from 2006 to 2021. The Southern Tropical Indian Ocean (STIO) sea surface temperature anomaly (SSTA) in May has begun to warm at an accelerated pace since 2001, establishing a significant positive correlation with the Northwest Pacific anomalous anticyclone (WNPAC). Since 2004, the trend of WNAPAC in May has changed from significant weakening to significant strengthening, and in 2005 it has been significantly correlated with the abnormal increase of precipitation in Jiangnan in May, and the warm SSTA of southern tropical India associated with WNAPAC has lasted from February to May. Since 2006, while the WNAPAC has intensified, the anomalous convection over the STIO has developed eastward, and the resulting updraft has sunk over the Philippine Sea, the South China Sea and the Indochina Peninsula with the Hadley Circulation, forming a more pronounced convective dipole structure than in 1980–2005. The downdraft has strengthened the WNPAC, which has promoted the transport of water vapor to Jiangnan, and the thermal component of the anomalous precipitation in Jiangnan in May has increased. After the enhancement of WNPAC, the secondary circulation between the active area of WNPAC and Jiangnan is more compact, the uplift of Jiangnan is enhanced, and the dynamic component of anomalous precipitation increases. The STIO warm SSTA, which is associated with the increase in abnormal precipitation in Jiangnan in May, has been maintained from March to May since 2006, and WNAPAC has played a role as a link. If the trends change are removed, it is difficult for the SSTA in STIO to affect the abnormal precipitation in Jiangnan in May through WNAPAC since 2006.