A dynamical and mesoscale analysis of a heavy rainfall event on 10 July 2004 in Beijing is made with the routine and densified observation, NCEP data and radar images. It's found that the heavy rainfall occurred under the favorable environmental conditions. Three rain clusters that brought heavy rainfall have the features of meso-β system and the convective echoes are the direct producer. The mesoscale convergence line and the mesoscale low provided favorable triggering effect for the occurrence of the heavy rainfall.