By using a regional model named AREM (Advanced Regional Eta-coordinate Model), which was developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamic, Institute of Atmospheric Physics, Chinese Academy of Sciences, several local heavy rainfall events happening over the western Sichuan basin during August 2003 are simulated and analyzed. The results indicate that the initial local water vapor condition is crucial to the happening and evolution of heavy rainfall events over the western Sichuan basin. Both the intensity of heavy rainfall and the time of maximum precipitation are highly sensitive to the initial condition of local water vapor. An increase of initial local water vapor would not only increase the 24-hour accumulated precipitation amount, but also lead to the main rainfall events happening earlier, resulting in a day-time rather than nocturnal rainfall events. Correspondingly, a decrease of initial water vapor would delay the maximum precipitation time and decrease the total precipitation amount.