The rainstorm with rainfall 226 mm in 6 h in the downstream of Changjiang River Valley on 27 July 2009 is simulated by WRF model. According to the model output, the diagnostic analysis is done for the shallow low vortex rainstorm event to investigate the development mechanism of the system. The results show that the low vortex locates at forepart of westerlies trough, existing only in the low level of troposphere. The dynamic structure and temperature moisture features all show that the vortex is a shallow system. The low vortex rainfall mainly happens near the warm shear line of the east part of the vortex. Two moisture source regions are in the South China Sea and the East Sea, and the thick moisture convergence is up to 600 hPa. The low vortex has strong baroclinic structure features. The analysis of vorticity equation shows that the divergence and convergence of wind field contribute mainly to the positive vorticity change rate, and the 3D wind shear distribution has an important correlation with vorticity. The dynamics of terrain obstruct makes the low vortex strengthened at leeward slope. The jets at high and low levels and the latent heat of condensation are helpful to the intensification of the low vortex and the torrential rainfall and the eastward motion. The wet isentropic surface perpendicular to the vortex surface means the low vortex is at mature stage, which predicts the rainfall is to weaken soon. When the developing condition of slantwise vorticity (Cd>0) keeps strengthening, it indicates that the combined result of baroclinicity, stability and wind vertical shear is also an important impact factor for the shallow low vortex developing and moving.