In this paper, the physical features of a severe convective heavy rain, which occurs during the Meiyu period in the reaches of the Changjiang River and is difficult to forecast, are studied by the control analysis method. In such a severe convective heavy rain oecuring in the warm section far from the front, the upper divergence and lower convergence seldom move synchronously. When the upper divergence moving eastward along the edge of the subtropical high overlaps the lower quasi-static convergence, there occurs a heavy rainfall, and when the upper divergence moves away from the convergence, the heavy rain stops. Thus, attention should be paid to the divergence field above 200 hPa as well as the conditions of the lower convergence so that good results can be achieved for forecasting such heavy rain.