Abstract: Using conventional observation data, regional automatic weather station data, black body temperature (T_BB) from FY-2C satellite and NCEP reanalysis data with 1°×1° resolution, we have conducted the diagnostic analysis of a torrential rain event occurred along typical western Meiyu front in Qiandongnan from June 17 to 18 in 2015. The results indicate that (1) under the single blocking high background of two-trough and one-ridge pattern in Meiyu season, cold air moves southwards along the east side of Baikal Lake blocking high and counters warm and wet airflow from the South China Sea and the bay of Bengal in Qiandongnan. East-moving short-wave trough at 500 hPa makes the shear line in the low-level move eastwards and southwards and the Meiyu front in ground level develop, which, combined with the suction effect near the east ridge of the South Asian high at 200 hPa, triggers mesoscale convection system (MCS) that results in torrential rain. (2) The torrential rain, which mainly occurs from strengthening to mature stage of convective clouds, is caused by the genesis, eastward movement, merging and strengthening of multiple MCSs. The decreasing period of T_BB is basically consistent with the increasing phase of rainfall. (3) A α-mesoscale convection system embedded with several γ-or β-mesoscale cloud clusters in the Meiyu front rain zone maintains in Qiandongnan, which is the direct reason that severe precipitation can last in Qingshuijiang river basin. (4) The downward movement of both the positive helicity center in the low-level and the negative one in the mid-and upper-level, and the rapid increase of the positive helicity in the low-level are conductive to the rapid development of shear line in the low-level and the enhancement of precipitation. (5) The water vapor flux convergence zone at 850 hPa tallies well with the severe precipitation areas.