Abstract: Because of the resistance of large-area mountainous forest to air flow, there would be vertical wind speed shear formed over the forest, which would be much larger than that produced by the horizontal temperature gradient, and, would play an important role in triggering off the baroclinic wave instability. The influence of the vertical wind speed shear on the wave instability is discussed by the use of a two-layer numerical model. It is poiuted out that this influence is one of the main reasons for the rainfall increase going up with the increase of the height of the forest in the mountainous area.The vertical wind speed shear would vary with the change of coefficients of the ventilation in forest. The shear caused by dense and large-area forest would be larger than that by sparse forest; therefore, the increment of precipitation in the former would be heavier than in the latter. Seeing that a large-scale clear cutting in the primeval forest would break the normal movement pattern of airflow over the forest and cause decrease of rainfall, and thus result in the maladjustment of forest ecosystem, this paper urges that in order to maintain ecological equilibrium, reasonable felling and carefully exploitation of forest resoures must be strictly observed.