The effects of the terrain and terrain gradient on the heavy rain of Xichang are studied.It shows that the heavy rain on June 23,2004 is a southwest vortex-shear pattern, and the heavy rain on June 27,2004 is a two-high-convergence-pattern.With the mesoscale model (MM5V3.7) from PSU/NCAR,the two events are simulated.It is found that the mesoscale mountain has significant influence on the location and intensity of the heavy rain area,and the bell-mouthed area and the riverway area of Xichang are favorable to the occur-rence and development of mesoscale disturbance and heavy rain.