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