A precipitation process occurring during the opening ceremony of Olympic Games in Beijing was simulated by WRF v3.6.1 model coupled with Thompson cloud microphysical scheme, moreover, the effects of high,median and low Cloud Condensation Nucleus(CCN) concentration on rainfall were analyzed by comparing three groups of simulations. It was found that:(1) In the case of low concentration of CCN, the increasing of CCN concentration results in the enhanced amount of 24 h accumulated precipitation with a relative low extent, while in the case of high concentration of CCN, it is to the contrary. (2) From the distribution of rainfall intensity, the effects of CCN concentration variation on hard, heavy, moderate, light rain are both reflected in rainfall intensity,but not in precipitation phase. (3) The effects of CCN concentration on low cloud fraction are the same with the effects of it on rainfall, so the low cloud fraction is a key cause of the precipitation variation.