Abstract: The onset of the South China Sea Summer Monsoon (SCSSM) is one of the most critical factors affecting the formation and intensification of the Dragon-boat Rainfall in South China. Investigating the impacts and mechanisms of anomalous monsoon onset timing on the precipitation and intensity of the Dragon-boat Rainfall in South China can provides technical support for relevant meteorological services and climate prediction. Based on the SCSSM onset monitoring data from the National Climate Center for 1961—2025, daily precipitation data from 192 national meteorological stations across South China, daily NCEP/NCAR atmospheric reanalysis data, and monthly Sea Surface Temperature (SST) data, we first analyze the relationship of the anomalous onset timing of the SCSSM with precipitation as well as frequency and intensity of heavy rain events during the Dragon-boat Rainfall period in South China. Using correlation and composite methods, we then analyze the evolution characteristics of atmospheric circulation and water vapor transport during the Dragon-boat Rainfall period and the preceding period. Anomalous SST characteristics in the preceding winter and spring for early-onset years (6 a), late-onset years (10 a), and typical early-onset years (1996, 2008, 2011) are also analyzed. The results show that over the past 65 years, the SCSSM onset date has exhibited an advancing trend and is negatively correlated with precipitation during the Dragon-boat Rainfall period (May 21 to June 20) in South China. Both the advancing trend and the negative correlation have become more pronounced after 1994. In early-onset years, South China is more prone to high-intensity precipitation events during the Dragon-boat Rainfall period; in late-onset years, heavy rain events are predominantly of low intensity. In early-onset years, during pentads 25—29, the East Asian trough is stronger, accompanied by enhanced cold air activities; during pentad 28, the Western Pacific Subtropical High (WPSH) weakens significantly, while low-level winds over the South China Sea shift from easterly to westerly, favoring the transport of warm and moist westerly—southwesterly airflows from the Arabian Sea, the Bay of Bengal, the Indochina Peninsula and the South China Sea to South China, with northerly anomalies over northern South China. During the Dragon-boat Rainfall period in early-onset years, the upper-level South Asian high is significantly weaker, the mid-level geopotential height over central and eastern China is markedly lower, and the East Asian trough is stronger. At lower levels, an anticyclonic circulation to the east of the Philippines is favorable for water vapor transport from the western Pacific and South China Sea to South China, where the convergence of southerly and northerly airflows results in moisture convergence, and vertical motion over South China is enhanced. This configuration of upper- and lower-level circulation promotes significantly increased precipitation over South China during the Dragon-boat Rainfall period. The opposite is true for late-onset years. In the early-onset years of 1996 and 2011, the SCSSM experienced interruptions during pentads 27 and 28, respectively. During the Dragon-boat Rainfall period of these two years, the WPSH strengthened notably compared to the preceding period, while water vapor transport from the Bay of Bengal to the South China Sea weakened significantly, the East Asian trough weakened markedly, and the cold air activity was anomalously weak with no convergence of cold and warm air over South China. As a result, anomalously low Dragon-boat Rainfall occurred in 1996 and 2011. In the early-onset year of 2008, anomalously strong westerly—southwesterly water vapor transport from the Arabian Sea, the Bay of Bengal, the Indochina Peninsula and the South China Sea persisted after the onset. During the Dragon-boat Rainfall period, the WPSH weakened and retreated eastward, and cold air moved southward, with cold and warm air converging over South China, resulting in anomalously high Dragon-boat Rainfall in 2008. In the preceding winter and spring of early-onset years, SST anomalies in the equatorial central-eastern Pacific exhibited a La Niña pattern, with significantly high SST in the South China Sea and low SST in the equatorial Indian Ocean and the northwestern Pacific to the east of the Okhotsk Sea. This pattern is favorable for an early SCSSM onset and stronger water vapor transport to the South China Sea and South China, and thereby contributes to increased precipitation during the Dragon-boat Rainfall period in South China. In summary, this study has yielded meaningful results regarding possible impacts of the early or late onset of the South China Sea summer monsoon on the Dragon-Boat Rainfall over South China, providing technical references for related operational forecasting and services.