A rare torrential rain with 244.7 mm rainfall in its storm centre has taken place in the north of Jiangsu province from September 2 to 3 in 2010. Analysis shows that this torrential rain process happened in the inverted trough of TS named “Lionrock”, which landed on South China Sea, while warm and wet flow met the colder airmass in the process of northward moving at the bottom of the troposphere. Thus local frontogenesis has occurred in storm area and the dry cold air inclined downward from the middlelevel，which reinforced the baroclinicity of atmosphere. This not only caused convective instability, but also supplied the power to lift the warm and wet flow. So that cyclonic vorticity developed along the frontal zone. In this torrential rain process the southerly in the east of inverted trough of westward moving STS and the southern airflow from the east of subtropical high have formed a compensatory lower jet stream, which carried large abundant warm and wet flow to storm area.