Abstract: Based on the products of the mesoscale numerical model MM4 and surface observational data,the dynamical structure of a typical mesoscale shear-line rain belt occurring on Meiyufront in the Changjiang-Huaihe River Basin during July 8-9,1991 has been analyzed in this paper.The results showed that there was a mesoscale vortex tube along the mesoscale shearline on the middle-low troposphere,and its altitude varied in a wave-shape.There was stronger longitudinal circulation near the wave peak.The surface mesoscale cyclonic and anti-cyclonic circulations were located at east and west sides of the wave peak respectively.Heavy rain area located under the vertical longitudinal circulation near the wave peak.The precipitation intensity was closely related with the intensity of the vortextube.The latter was closely related with the conditional symmetric instability in the middle-low troposphere.