Abstract: Based on the results of the numerical simulation for a heavy rainfall case occurring on May 23, 1998 during the IOP period of HUAMEX, the relation of front-related vertical circulation with convection was investigated. In addition to the convection trigger mechanism provided by cold front, there is a possible interaction between cold front and MCS. In case convection occurs on front, the structure of vertical circulation on the cross section of front may change significantly. In stead of sliding upward along the slant front, the warm and wet stream in the MCS in front of the cold front ascents to the upper air directly. However, in case that there are MCSs occurring on both sides of the cold front simultaneously, there are stream coming from the rear of the cold front and penetrate it. The trajectory of the air mass from the convection also reveals the phenomena. These results reveal that the front is not an exact ‘material’ surface as its definition. Actually, it is just an interface in atmosphere with significantly different temperature and moisture attributions. In the pre-flooding season in South China, the temperature difference is not so obvious as that of common sense, which could be regarded as a reason of the existence of the penetrating upstream of front. MCS should be able to affect the vertical stream structure of cold front due to its similar scale with that of front on cross front direction.