Abstract: The formation mechanism of an extreme rainstorm process in Yunnan on October 9th, 2015 was analyzed using FNL reanalysis data, conventional observation, and FY-2E satellite observatory data. Results showed that this extreme rainstorm in Yunnan was caused by the Bay of Bengal low combined with Kunming quasi-stationary front. The Bay of Bengal low maintained for a long time after landing, which provided long-lasting and abundant supply of water vapor. The shear line at 700 hPa and Kunming quasi-stationary front greatly enhanced the water vapor convergence. Rising movement was strongly developed because of high-level and low-level jet streams and frontal lifting. The slantwise updraft by the orographic uplift and frontal lift was favorable to the release of conditional symmetric unstable energy in moisture air.