In this paper, a process of shear line storm rainfall, which located in the valley of middle and lower Yangtze River and happened from 12th to 13th on June 1998, is diagnostically studied. At the same time, a numerical experiment of this process is made, using a fine-mesh η-coordinate storm rainfall numer ical model. It is found that the β-mesoscale system, which is related with thi s process, mainly shows the shear line on the low level of troposphere and the β-mesoscale cyclone on surface. The eastern move and development of the cyclon e brings the wide area intensive precipitation in the valley of middle and lower Yangtze River. Potential instability plays a crucial role in starting this stor m rainfall. Deep β-mesoscale vortex pole is a distinct characteristic of this storm rain-mass. Topography has important influence on both the intensity and area of storm rainfall. Inten sity of the shear line also has evident effect on rainfall quantity in the proce ss of storm rainfall.