Abstract: Based on weather radar data, precipitation data from densified rainfall stations, hydrological data at Wucha hydrological station in Qijiang Catchment and ERA5 reanalysis data, we have conducted the analysis of a record-breaking flood event occurred in Qijiang Catchment from June 19 to 22 in 2020 including the spatial-temporal distribution of precipitation, the circulation background, the water vapor sources and the evolutional characteristics of mesoscale convective systems (MCSs). The results indicate that (1) this event is a persistent severe precipitation event with concentrated range and characterized by obvious periodic and diurnal variation. It can be divided into three stages. Severe precipitation mainly occurs at night. (2) Persistent heavy rainfall occurs under the circulation pattern of cold air guided by east-moving cold low pressure in the Ural Mountains moving southward to the northeast of Sichuan Basin, low trough in the mid-latitude maintaining for a long time in the eastern Sichuan-Chongqing-Guizhou area and the Western Pacific subtropical high being stable. (3) Low-level warm and moist southwest jet further increases the atmospheric stratification instability between northern Guizhou and southern Chongqing, and it conveys water vapor from South China Sea and Bengal Bay to Qijiang River Basin, providing abundant water vapor for the generation and persistence of heavy rainfall. (4) The occurrence and development of MCSs at the three severe precipitation stages show different features. At the first stage, MCSs in coherence with the strong radar echoes are situated in roughly north-south direction, moving eastward to affect the Qijiang Catchment, and the duration of heavy rainfall is therefore relatively short. Next, the MCSs present southwest-northeast direction, affecting Qijiang Catchment for a longer time, but the echo intensity is weak. At last stage, the MCSs present massive. And they are linearly distributed in the northwest-southeast direction when they mature, exhibiting "train effect", which affects Qijiang Catchment longest and cause the greater precipitation intensity.