For the squall line process occurred on 28 April 2006 in Jiangsu Province, the main characteristics, including its boundary layer, vertical structure, propagation evolution, and development, are analyzed. The physical causes of the surface strong wind caused by the squall line are also discussed. The results show that the squall line was activated and developed along a surface convergence line, when the squall line propagated downstream; it generated perturbation in horizontal wind field in the boundary layer, resulting in obvious disturbance features accompanying the squall line propagation. In addition, the drag effect of rain particles, in cooperation with the down stream at the back of squall line, played a crucial role in generating the surface strong wind.