Abstract: Boundary layer is the major source of motivity and vapor for meso-scale convective systems(MCS). A severe storm event in the mid west of Liaoning province on 5 July 2004 was simulated using the MM5(V3.6) coupled with the Noar LSM, to explore the boundary layer characteristics, and the triggering and maintaining mechanism of a MCS in the cold vortex process over Northeast China. The 3-D airflow structure, cold air dome, stratification structure of the boundary layer and their effects on the triggering and maintaining of the MCS were emphatically analyzed. The conclusions are as follow: there were 3 airflows converging in the area of the MCS; the first was the northeast flow from a stable air mass over the Changbai Mountains, the second was the northwest sinking flow from northwest Liaoning province and the third was a southwest flow. The shallow cold air in the bottom of the boundary layer in the northeast was favorable to the rise of the southwest flow and the generation of convection. In the initial stage, the convergence of the low layer was stronger than that of the upper layer of the boundary layer, and the rotation of the boundary layer flow was weak, so a convergence line of the boundary layer was one of important factors to trigger convection. In accompany with development of strong convection, a mesoscale vortex formed within the MCS at the upper boundary layer driven by convergence of the environmental wind field and Ekman pumping; it on one hand was the major entry for the inflow of the MCS, and on the other hand provided an important feedback to the maintain and development of convection. The cold air dome resulted from precipitation evaporation led to the formation of the stable stratification in the near surface layer, and the evolution of its temperature and moist stratifications and flow structure changed the inflow of the MCS from the boundary layer.