A rainstorm occurring in Shanxi Province on 13 Jun 2007 is simulated using the Weather Research and Forecasting (WRF) mesoscale model coupled with the Morrisions double-moment bulk microphysical scheme. Sensitivity tests are conducted and compared for different background conditions to understand the response of cloud microphysical processes and precipitation to changes in concentrations of aerosol particles. The results show that the total rainfall amount is reduced by about 0.8% in the polluted case as compared with the clean case, but a more intensive rainfall rate in the central region of the rain area and no significant change in the rainfall area are observed in the former case than in the latter. Snow crystals and graupel are the main sources for rain at the surface. In the polluted case considered here, higher concentration of aerosol particles leads to the decrease of warm rain at the early stage, and consequently more numerous snow particles can grow by coagulations with cloud droplets and rain droplets in the air for longer, intensifying precipitation at the later stage.