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Based on the automatic weather station, Doppler radar, FY4A, ECMWF model and NCEP reanalysis data, the extraordinary rain process from July 17 to 19, 2020, is analyzed. The results are as followings. The extraordinary rain occurred near Dabie Mountain and Lujiang County in Anhui Province, caused by the quasistationary mesoscale convective system (MCS) and the quasistationary vortex cell in MCS. The mesoscale cyclonic disturbance was composed of the shear line and cyclone. Under the background of high energy and strong instability, the extraordinary rain was caused by the mesoscale cyclone in the middle and east. In mesoscale cyclone propagation, the cell continuously produced, merged and strengthened, and moved slowly. The factors such as jet stream, convergence, dry intrusion and vertical circulation played an important role in the development and evolution of organized MCS. The southwest jet from the southern side of the lowlevel shear line to South China transported water vapour to the Yangtze and Huaihe River Basin, where there was strong convergence. There were jets at the high, low, and ultra-low levels, and the coupling of high and lowlevel jets intensified the strong development of MCS. The surface convergence line was the trigger mechanism of MCS in the early stage. With the invasion of dry and cold air, the baroclinity of the atmosphere and the convective instability of MCS were increased. The surface convergence line was the trigger mechanism of MCS in the early stage. With the invasion of dry and cold air, the baroclinity of the atmosphere and the convective instability of MCS were increased. There were vertical circulation circles on both sides of the Meiyu front, that was, the convective heating between the convection and the upper jet generated thermal wind adjustment at the entrance of the upper jet stream to maintain the development and evolution of the Meiyu front, and the strong ascending and descending motion promoted the strengthening and falling of MCS and continuous occurrence of rain. The terrain uplift of Dabie Mountain and the narrow pipe effect in the upper reaches induced extraordinary rain in the two places.