A severe storm caused by northeast cold vortex over Liaoning Province on 12 July 2002,was successfully simulated by MM5 model. Middle-level dry cold air adiabatic subsiding is the important mechanism of 700hPa dry-warm lid establishment and maintenance. Both the Warm moist ascending airflow and the dry warm lid restraining are the important mechanism of convective instable energy accumulation. The differential advection which is produced by vertical wind shear from southwest at low-level to the northwest at the middle-level enhances the stratification instability before the storm approach,and the rapid anti-clockwise rotation of wind vertical shear when the storm approach that made thermal wind imbalance and inspired vertical circulation to adapt it's change,and it is important to the development of the storm.