Abstract: The heavy rain event occurred in Liuzhou city on 2-3 July 2009 is numerically simulated by the WRF model. The result of simulation conforms to the reality. By conducting terrain sensitivity test, the effect of local mesoscale terrain on the torrential rain and its mechanism are studied. The result shows that the terrain plays an important role in the formation of the heavy rain although it is not the decisive factor. Mountain makes the water vapor and heat that are brought by southwest warm moist flow accumulate at wind ward slope, which in turn increases the convective instability over area of Rongan and Rongshui. The vertical ascending motion excited by vertical disturbance is enhanced as low-level cold air from northwest around the mountains invades. It then triggers the development of convective instability. When the terrain is reduced to “platform”, precipitation near the mountain decreases. Analysis of physical variable fields also shows that upward motion and vorticity are weaker than in the case of having terrain because of lack the uplift effects of terrain.