Abstract: By using all available observational data, such as satellite, Doppler radar images as well as routine observational data, and non-hydrostatic model MM5V3.5, a severe MCS (Mesoscale Convective System) occurred on 23 August 2001 in the North China was investigated. The evolutionary process and development mechanism of this MCS case was analyzed with emphasis on the orographic effect of Taihang Mountain on its development. The observational data show this severe storm occurred in the pattern of high-level northwest flow, the temperature increase in the lower troposphere descends in high-mid troposphere and the humidity enhanced in the whole troposphere induced the convective instability. The simulation results indicated that the downhill flow form Taihang Mountain is the key mechanism to initiate the convection. The characteristics of moist potential vorticity (MPV) show that the moist isentropes slant is the foundation of the convection, the vertical and horizontal components, i.e. pm1 and p m2 respectively, pm10 in the lower of troposphere are favorable condition for the development of MSC, it can serve a s a powerful tools for the diagnosis and prediction of MCS. On the other hand, because the condition of MPV conservation can be destroyed possibly in the MCS process, for example the hydrostatic balance and the mass forcing induced by the precipitation et al., canlead to MPV anomaly. In the early period of the MCS，the vertical second circulation can be an important mechanism to trigger new thund erstorm and conduce the storm spreading by jump model. The Taihang Mountain can change the wind field in boundary layer; include downhill flow, vertical shear o f wind and mesoscale convergence line. As a result, it plays an important role in initiating, organization and movement of the storm.