Abstract: In this article, three cases of heavy rains which occurred in the Jing-Jin-Ji area of China have been analysed. The results show: 1. These heavy rains are affected by the upper trough with cold front. 2. Ten hours before the rains there exist high energy tongues of moist available energy in the southwest of the rain area and the convergence of energy and vapor in the north and west of the high tongues. 3. The frontgenesis of the lower troposphere and the lifting role of the mountain are the direct triggering mechanisms.