Abstract: Extreme severe rain occurred in northeastern Hubei Province for two successive days on 12-13 July 2012, with the maximum total rainfall of 600 mm. Using the satellite data, radar data and Automatic Weather Station (AWS) data, the extreme severe rain occurred on the southwest side of the Dabie Mountain on 12 July 2012 is comprehensively analyzed focusing on the occurrence and development of Mescoscale Convective System (MCS), as well as its structure characteristics and propagation features. The results show that: (1) The back-building and quasi-stationary meso-α-scale convective system (BB) exactly result in the extreme severe rain in northeastern Hubei Province on 12 July 2012. (2) This kind of MCS frequently generates in the northeast of Hubei Province, and closely related to the thunderstorm cold pool blocked by the Dabie Mountain in the east side of Hubei Province. The cold air piles up in front of the mountain and generates backflow because of the blocking, leading to the MCS spreading from downward to upwind. (3) The back-propagating MCS rarely moves or causes obvious train effect, which bring about extreme severe rain before the mountain in a certain distance. And (4) the cold outflow boundary of the thunderstorm moves upwind, giving rise to the back propagation of convection cell, which is one of the important formation mechanisms for the fingerlike convective cluster.