Using data of sounding, surface automatic weather stations, and doppler radar, as well as NCEP/NCAR reanalysis data, a severe squall line process that affected the greater part of Shandong Province on 28 April 2006 is diagnosed and analyzed. The severe squall line was generated by convections triggered by a moving transversal trough of a northeast vortex. The backward-tilting dry and cold downdraft dominated the bottom layer of the transversal trough, and the dry intrusion was favorable for accumulation of unstable energy. The transversal trough was the strongest at 300 hPa, connecting southern and northern upper jet streams with obvious tropopause folding; the geostrophic adjustment of the upper transversal trough induced mesoscale gravity waves (MGWs), which promoted the downward transmission of upper momentum, acting as an important dynamic source for severe surface wind. The storm area had characteristic of slantwise isentropic surface, convective instability, and vertical wind shear, which satisfied the key condition of slantwise vorticity development. In the mature period of the squall line, the divergence, vertical velocity, and the perturbations of surface wind and pressure showed the characteristics of typical MGWs in the lower and middle layer of troposphere. The storm and MGWs forms a self-developed system, and it is a key dynamic mechanism to enhance the squall line.