Abstract: May–June marks a critical transition from the dry season to the rainy season in Yunnan, during which precipitation significantly influences local agricultural production and the ecological environment. It also serves as an important indicator of the onset time of the rainy season. This study examines precipitation variability in Yunnan during May–June utilizing observational precipitation data from 1971 to 2022, along with concurrent NCEP/NCAR reanalysis data. Our analysis reveals a significant decreasing trend in cumulative precipitation in southwestern Yunnan from May to June, with a climate change point identified in 2009 and a marked reduction after 2010. Further investigation indicates that the warm anomalous sea surface temperature (SST) in the tropical western Indian Ocean weakens divergence at 200 hPa and convergence at 700 hPa over the northern Indo-China Peninsula and Yunnan, hindering vertical atmospheric uplift and precipitation. Additionally, a positive anomaly in the 500-hPa height field over central Asia and the Bay of Bengal reduces the cold air from mid-to-high latitudes and the southwest airflow ahead of the southern branch trough, thereby inhibiting precipitation processes in southwestern Yunnan. Concurrently, an anomalous anticyclonic circulation from the South China Sea to the Indo-China Peninsula in the lower atmosphere generates anomalous northwest winds over Yunnan, weakening moisture transport by southwest winds and further contributing to the decrease in precipitation. The continuous increase in the SST of the tropical western Indian Ocean and its abrupt warming in the late 2010s are identified as key drivers for the sudden decrease in precipitation in southwestern Yunnan during May–June.