Abstract: Precipitation in May in Jiangnan has shown an overall upward trend since 1980. Considering this observation, this study considers the precipitation in May in Jiangnan from 1980 to 2021 as the research object. The Climate Prediction Center (CPC) precipitation data, National Centers for Environmental Prediction (NCEP) atmospheric circulation data, and Hadley Center sea surface temperature data were used to analyze the precipitation trend characteristics in May in Jiangnan, diagnose the water vapor budget, and finally discuss the possible physical processes. The results revealed that the dynamic component of precipitation in Jiangnan in May is one order of magnitude larger than the thermal component. Both components showed no obvious trend change from 1980 to 2005, but showed significant increasing trends from 2006 to 2021. The Southern Tropical Indian Ocean (STIO) Sea Surface Temperature Anomaly (SSTA) in May began rising at an accelerated pace since 2001, establishing a significant positive correlation with WNPAC (Northwest Pacific anomalous anticyclone). Since 2004, the trend of WNAPAC in May changed from significant weakening to significant strengthening, and in 2005, it was significantly correlated with the abnormal increase in precipitation in Jiangnan in May. The warm SSTA of southern tropical India associated with WNAPAC lasted from February to May 2005. Since 2006, while the WNAPAC intensified, the anomalous convection over the STIO developed eastward. The resulting updraft sank over the Philippine Sea, the South China Sea, and the Indochina Peninsula with the Hadley Circulation, forming a more pronounced convective dipole structure than that existing in the period 1980–2005. The downdraft has strengthened the WNPAC, thus promoting water vapor transport to Jiangnan, and the thermal component of anomalous precipitation in Jiangnan in May has increased. After WNPAC was enhanced, the secondary circulation between the active area of WNPAC and Jiangnan became more compact, and the uplift in Jiangnan was enhanced, accompanied by an increase in the dynamic component of anomalous precipitation. 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 is an important link. If the change in trends is removed, the SSTA in STIO does not affect the abnormal precipitation in Jiangnan in May through WNAPAC since 2006.