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Using data from 373 meteorological stations in the Western China region and ERA5 reanalysis data from 1981 to 2022, this study employs methods such as Mann-Kendall, correlation, and composites to analyse the characteristics of autumn rainfall changes in Western China after entering a new climatic phase, and explores the atmospheric circulation and sea temperature anomaly characteristics. The results indicate that over the past 32 years, the overall trend of Western China autumn rain significantly increases, with an increase in precipitation of approximately 30.4 mm, at a rate of about 9.5 mm per decade. Spatially, most areas show an increase post-shift, with significant increases primarily located in most of the Sichuan Basin, western Hunan, southern Shanxi, and southern Ningxia regions. Circulation field analysis reveals that since 2011, the blocking high near the western coast of Europe to the Ural Mountains and the Sea of Okhotsk area (hereafter referred to as the Okhotsk high) strengthens. The Western Pacific Subtropical High (hereafter referred to as the West Pacific Sub-high) also intensifies and extends westward. The southwesterly winds turning through the South China Sea from the subtropical region of the Western Pacific, the easterlies from the Northwest Pacific region, and the southwesterly winds from the Indian Ocean through the Bay of Bengal all strengthen. These factors collectively contribute to an increasing trend in water vapour transported to Southwestern, providing favourable conditions for the transition from lesser to greater autumn rainfall. Sea surface field analysis indicates that during the earlier spring and summer, as well as the concurrent autumn seasons, sea temperatures in the Indian Ocean transition from cold to warm, while sea temperatures in the central Pacific shift from warm to cold. These shifts prompt adjustments in convective activity and meridional circulation across the mid-low latitudes. These also lead to a shift in the West Pacific Sub-high from weak to strong, markedly extending westward, enhancing the intensity of water vapour transported from its western side and the Bay of Bengal to Southwestern. Anomalous warming in the Northwest Pacific Ocean, by stimulating meridional teleconnection wave trains, strengthens the Okhotsk high. Elevated sea temperatures in the western Atlantic increase convective activity from the Arabian Sea to the Bay of Bengal, which, through downstream propagation of teleconnection wave trains, facilitates the transport of water vapour from the Bay of Bengal to Southwestern. The combined sea temperature changes in these regions thus intensify the water vapour transport from the western side of the West Pacific Sub-high and the Bay of Bengal to Southwestern, consequently leading to an increase in autumn rainfall from lesser to greater amounts.