Using the non-hydrostatic mesoscale model MM5,the rainstorm caused by the interaction between typhoon Aili and cold air in Shandong Province on 26-28 August 2004 is simulated,the formation mechanism of the rainstorm is analyzed.The results show that the jet stream in the peripheral of the typhoon led to deep moist air layer and intense water vapor convergence over the area from the East China Coast to the South of Shandong,i.e.the accumulation of unstable energy.The intrusion of dry/cold air at the upper levels over the rainfall areas resulted in intensified instability,and the saturation of unsaturaded moist air,thus enhancing the rainfall.The intensification of the positive vorticity and divergence at the low levels induced the formation of the meso-scale vortex and the center of torrential rainfall.