Abstract: There is always a meridionally secondary vertical circulation system over the rain belt of a meiyu front (or a shear line) in Chinese continent during summer season.Authors call the system "monsoon circulation tube". The physical mechanisms of formation and maintenance of the "monsoon circulation tube" are then invesstigated from the relationships between it and diabatic heating and also thermal and dynamic effects of plateau topography. It is also discussed that the spatial relations between the "mosoon circulation tube" and planetary scale systems, such as the westerly, the easterly and the subtropic high, and their effects on the "monsoon circulation tube".