By using the ETA model, a numerical simulations was done for a sharp heavy rain in the west Sichuan basin on 24 August, 1995 and the numerical experiments without shear line over Qinghai -Xizang Plateau and Xichang small high were also done for the heavy rain. The results of numerical experiments are as follws: (1) The activity of the shear line over the Plateau can strengthen the heavy rain in west Sichuan Basin, but it will be weaken when there is the Xichang small high. (2) The main mechanism of strengthening heavy rain caused by the activity of the shear line over the Plateau is that strengthening of the convergence of vapour flux and flow field, positive vorticity, ascending motion in lower troposphere and convergence of vapour flux and flow field in middle troposphere over the heavy rain area. (3) The vapour and dynamic conditions in lower troposphere are very important for the development of heavy rain.