The heavy rain in north-west region of Hunan province is analyzed. Result shows as follows. The heavy rain is related to the southwest jet at the low and middle troposphere and the block in the high latitude area of the Eurasia. The maintaining block keeps two cyclones motionless at the middle troposphere, so the northeast dry and cold air develops and meets the warm and wet air from southwest region, and the northeast and southwest shear line came into being. Evolution and development of the deep mesoscale cyclone on the shear line is the reason why the rain becomes so heavy. As the big vorticity and the big diver of Q-vector in the low troposphere and the potential vorticity on 345K isentropic surface move from south to north, the heavy rain begins from south to north, so this characteristic can be used as the sign of forecasting heavy rain. The coupling of the strong diver and vorticity cylinder and the steep θ_se structure are the fundamental dynamic mechanism of vertical motion and the heavy rain cause and persistence.