The impact of vegetation coverage on the mesoscale boundary layer structure of the midlatitude semiarid area has been investigated by using a mesoscale biological meteorological model coupled with a cumulus ensemble parameterization scheme over both a flat terrain and a complicated terrain under conditions with－ and without－synoptic flow and different background atmospheric humidity. The investigation indicates that regional perturbations in temperature and specific humidity over vegetation covered area from an adjacent dry, hair soil, caused by the water transpiration from vegetation and more evaporation from the underlying soil surface as compared to the adjacent dry, bare soil, since the soil covered by vegetation will be humider as compared to the bare soil. As a result, a positive pressure perturbation was created over vegetation area. A mesoscale circulation (called as vegetation breeze circulation, VBC) is forced over a flat terrain under calm synoptic condition by the horizontal pressure gradient with a wind speed reaching about 3. 2 m/s, directing from the vegetation area to the bare soil in the lower boundary layer. The regional climate over a flat terrain is greatly impacted by the VBC. Simulations also indicated that the convective precipitations were initiated and organized by vegetation －breeze circulations. The timing, location and strength of the precipitations will probably be controlled by the convergence zone associated with VBC’s fronts and their interaction. Synoptic flow, atmospheric specific humidity and landscape changes played a significant role in modifying the structure and distribution of perturbations in wind, specific humidity, temperature and deep convective features (including convective clouds and precipitation). That is the reason why the deep convective precipitation event was considerably different day by day. The climate over a complex terrain is determined by the interactions between upslope flow circulation (UFC) and the VBC. Over a terrain defined as plain－slope－ plateau, when the plain is covered by vegetation the UFC will be enhanced because the UFC is in phase with the VBC; when the vegetation covers the slope and plateau area, the VBC is out of phase with UFC, the simulated UFC will decrease; when the terrain is completely covered by vegetation, the intensity of the thermally forced upslope flow circulation is decreased; when the terrain in the east of the plateau and west of the plain are covered by the canopy, the mesoscale climate will depend upon the interaction between VBC and UFC.