Abstract: On 24-25 June 2002, a torrential rain event occurred in Beijing near Mentougou, which is regarded as a typical orographic torrential rain event under the eastern flow in the boundary. In order to study the dynamic effect of the orography in this event, numerical simulation is conducted with the Advanced Regional Prediction System (ARPS). Control experiment twodomain oneway nested grids with the respective horizontal resolutions of 27 and 9 km, and covers squared domains of 3000×3000 and 900×900 km2, respectively. Both domains use the full physics mode of the ARPS and the terrain data are created from a 30second global terrain data base for the two domains. Three sets of sensitive experiments are conducted for comparison with the control experiment, the first one is dry simulation, that is, no latent heat is considered in the simulation; in the second one, the orographies, as a whole, are moved 1 degree easterly and westerly, respectively, and in the third one, parts of the orography in the southwest of Beijing near Mentougou are removed. The results show that the southeasterly airflow from the Bohai Sea is able to climb to the height of 2 km, which is over the LFC, testifying that the dynamic effect of the orography is the trigger mechanism in this torrential rain event. The convective instability of the air changes when moving the orography, as a result, the location and intensity of the simulated rainfall changes. This result shows that the convective instability of the air on the slope and over the mountain is necessary for the torrential rain to occur. The fact that the simulated rainfall changes as well after removing parts of the orography verifies that the torrential rain is caused under the effect of the multiscale orography and the suitable weather situation.