Abstract: In this paper the heavy rain on August 2,1976 in the central part of Gansu has been analysed.It took place when a trough of tropical easterlies moved in and developed on encountering a small depression from the Tibetan plateau.Accompanying the deep humid south-eastern flow in the lower troposphere,cold upper tropospheric air moved to Gansu and a deep layer of potential instability was produced and met the cold north-western flow later on.The convergence of vapour flux was introduced Analysis of hourly synoptic charts reveals that an intense subsynoptic depression is responsible for the heavy rain.The structure and life history of subsynoptic depression and the accompanying shear line are also discussed.When the shear line of low layer moved to the subsynoptic depression,the heavy rain was formed.