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An extraordinary heavy rain event caused by Typhoon Mangkhut occurred in the Yangtze River Delta region on 16 September 2018, with the maximum of 24-h accumulated rainfall at a single station reaching 297 mm. However, numerical models and subjective forecast failed to predict this typhoon remote rainstorm accurately. In this study, multiple observational data, an analysis dataset, and a trajectory model are used to analyze the causes of this severe rainstorm. The results show that the circulation situation provides a favorable large-scale background condition for the generation of the rainstorm. The coupling of the upper-level westerly jet and the low-level southerly jet is beneficial to the development of strong convections. In the rainstorm area there is a positive vorticity center connected to the main body of the typhoon. The cooling and humidifying effect of dry-cold air saturates the formerly unsaturated wet air, leading to the increase of precipitation. Besides, there is a lower-tropospheric moisture transport path connecting the typhoon and the rainstorm area, providing abundant moisture for the development of rainstorms. The backward trajectory simulation shows that the moisture mainly originates from the lower troposphere over the Philippine Sea, the southern South China Sea, and the sea south of the Philippines. [ABSTRACT FROM AUTHOR]