By using of the TBB data of 16 cases during 1981-1991,the influence of Bengal Bay depression on the heavy rainfall in Yunnan province was analysed.The isotherm,temperature gradient of cloud top,and the effectiveness of cloud precipitation were calculated.The structure and cloud images of the mesoscale convective system causing heavy rainfall were also analysed.The relationship of the cloud image characters of the depression and heavy rainfall in Yunnan was described.