Based on the multi-source observation and reanalysis data, the characteristics and main causes of the flood-causing rainstorm in May, 2019 in the middle-lower reaches of the Yangtze River were studied by using the synoptic diagnostic analysis method. The results indicated that the extreme rainstorm with the characteristics of mesoscale and stage was formed by the accumulation of continuously short-time heavy precipitation under the stable "low in the west and high in the east" weather background. The mesoscale convective system directly causing the extreme rainstorm was constantly regenerated and moved eastward in the front of the low-level jet with obvious "train effect" and quasi-static features. Continuously favorable water vapor and convective instability were provided by theextremely strong low-level jet and ultra-low-level jet. In particular, the high humidity of the whole layer, low lifting condensation height and large warm cloud thickness were conducive to improving theprecipitation efficiency and producing the short-term heavy precipitation with high rain intensity. The high- and low-level jet coupling was an important dynamic mechanism for the occurrence and development of heavy rainfall, and the strong vertical wind shear area has a good correspondence with the heavy rainfall area.