Abstract: In order to investigate the meso-scale orographic effects on heavy rain,we conduct a series of sensitivity experiments for a mei-yu case by using of a meso-scale model.It is found that the meso-scale mountain has significant influence on the location and intensity of heavy rain area,the center of heavy rain is located over the moutain area,the orographic increment of 12 hour accumulated rainfall accounts for about 90 per cent of the totals.The meso-scale mountain,as a foreign forcing,initiats cyclonic convergence and the concentration of heat and moisture in lower layer,then causes middle layer warming through latent heating,and high level anticyclonic divergence,which is favourable for the intensification and upward extension of the orographic vertical circulation.It is the positive feed-back mechanism among precipitation,latent heating and orographic vertical circulation that leads to the intense orographic increment of precipitation.Besides,the instability stratification caused by the afternoon ground heating also contributes to the development of orographic vertical circulation and the generation of new rainfall peak.The meso-scale disturbances on the initial fields seems not to play a significant role in the meso-scale orographic intensification of precipitation.