Abstract: A preliminary analysis of low-level dynamical and thermo-dynamical effects on a squall line case occurred in North China on 23 July 2009 is implemented, based on 4DVar assimilation to 6 CINRAD radar observations and numerical simulation with a three-dimensional cloud model under a rapid update cycling mode. The results indicate that the squall line is under the conditions of low-level moderate shear environments and the interaction of the low-level vertical wind shear with the cold pool is the key mechanism for structure and evolution of the squall line. There is strong low level vertical wind shear but weak cold pool during the initial period of squall line development with a ratio of C, the speed of the cold pool propagation, to ΔU, the component of the low-level shear perpendicular to the squall line, less than 1 (i.e., C/ΔU1) causes the squall line to gradually dissipate. The storm line echo observed by radars distinctly broadens and tilts upshear with very low height of strong echo top during the decay period of the squall line. Both the qualitative analysis and quantitative calculation from the simulated results show the influence explanation of the low-level shear and cold pool interaction on the squall line development is accordant to the RKW theory for addressing the squall line structure, evolution, and intensity by the relative balance between the cold pool intensity and the low-level shear magnitude that advanced by Rotunno and Weisman et al. In addition, simulated results indicate low-level 0-3 km shear is most important to squall line development whereas 0-6 km shear has also positive effects on squall line evolution, especially while the squall line is at its best survival state.