Abstract: The process of the heavy rain in the reaches of the Yangtze River in the east of Hubei Province on 21st July, 1998 is numerically simulated by a high-definition-coordinate model. The result of the simulation conforms to the reality. By sensitivity test and analysis, the effect of local mesoscale terrain on the torrential rain and the mechanism are studied. The result shows that the terrain plays an important part in the formation of the heavy rain although it is not the decisive factor. The southern warm and moist stream at boundary layer is obstructed by Dabie Mountain and the obstruction enforces the rain band at the upper part of the windward side. The water vapor at boundary layer is intercepted by Mubu Mountain which slows the initial development of the rain band at the lower part of the lee side. When the rain greatly develops, because there is a mesoscale low-level jet in the south of the rain band which just lies at Mubu Mountain, Ekman wind arrows lifted by the terrain at the frication layer turn around and point directly at the rain band and enforce the convergence of air and vapor. The divergence of the air stream at the ground layer during the period when the severe convective rain system develops at its highest degree, the rise of the converg ence layer that plays an important role in the maintenance of the heavy rain, the superposition of the frication convergence that is lifted by the terrain and enforces the intensity of the rainfall are the reasons why the extremely heavy rain lasts for a long time.