Abstract: Using composite method, the monsoon stream tube(MST)of early summer in 1983 has been analysed. It is indicated that over flood basins in the middle and lower Changjiang River and Huaihe River, a quite stable MST exists. It's variation is closely related not only to the position of the rainfall belt over Changjiang and Huaihe River, but also to the rainfall intensity. The classified composite analysis and the weak MST shows that the strong MST corresponds to the heavy rain corresponds to the weak rain.The calculation of heat sources indicates that the major motive forces which drive the MST are latent heat in the lower layar and sensible heat in the upper layer. The heat source is found at the ascending branch of the MST and the heat sink at the descending branch, consisting of a simple heat engine. It is significant to the formation of MST.