Based on conventional meteorological observation data, regional automatic weather station observation data, radiosonde data,Fy-2G data, precipitation fusion products and hourly reanalysis data of EC EAR 0.25° x 0.25°, the catastrophic rainstorm flood process in Qijiang area on June 22, 2020 was diagnosed and analyzed. The results show that the process occurs near the ridge line of the South Asian high, in front of the 500 hPa trough, between 700 hPa and 850 hPa vortex shear. Southerly warm and humid air between 850 hPa to 500 hPa continuously transports water vapor; near the surface layer. The cold air returning to the East forms a cold pad climbing. The convective cloud clusters generate, merge and develop to form a mesoscale convective system MCS, which has a strong indicative significance for heavy rainfall. The rainstorm area occurs in the cloud cluster merging development stage, and the cold cloud area is less than or equal to -52 ℃. The occurrence of short-term heavy rainfall is accompanied by the formation of ground convergence line, and the strongest hourly rain intensity center appears near the mesoscale convergence line and moves eastward slowly with the convergence line. The maximum accumulated rainfall area mainly occurs in Qijiang city and the upper reaches of Qihe River Basin.The station with the largest accumulated rainfall reaches 302.7 mm. The maximum hourly rainfall intensity is 64 mm·h^-1. The main reason for the rainstorm and flood disaster is that the hourly rainfall intensity is strong and the accumulated rainfall is large.