On the basis of the simulating results of the heavy rain occurred from July 29th to 30th, 1998 in the middle and lower reaches of the Yangtze River by means of MM5, the motion field structures of adjacent rainmasses are analyzed. The effect of the interaction of rainmasses of Meiyu front on their motion field structures is revealed. Due to the generation of new rainmass, and the anaflow structure of the adjacent one's changes from one kind of asymmetry to another type. The mesoscale vortex tube is wave-shaped not only in height, but also in the horizontal plane. Its wave shape in horizontal plane is associated with the location of rainmasses and the areas of strong and weak rainfall.