By use of synoptic map,cloud image,sounding data and Doppler radar data,the heavy rain gust case which occurred in the 23th of July,1998 is analyzed.It suggests that MCS can be developed on the downwind of MCC under positive large scale environment,which results in heavy rain gust of Shanghai.Furthermore,according to the Doppler radar base reflectivity,radial velocity and VAD Wind Profile (VWP) data closely related to the developing,strengthening,moving and merging of heavy rain gust echo,the forming and developing of trigger mechanism such as meso scale shear or meso scale eddy was detected several hours ago.