A few signs of occurrence and development of rneso-scale cloud cluster accompanied by heavy rainfall during midsummer afternoon over Beijing area had been found by the use of the moist available energy method. They are as follows :a) a lot of moist available energy was already stored up over Beijing area;b) the moist available energy is being accumulated over Beijing area; c) the flow pattern in the lower troposphere is favourable for the occurrence of the forced lifting;d) there will be a larger temperature contrast,which results from diabatic heating process,in the area between Beijing area and the bend of the Yellow River in the afternoon.