The paper focuses on analysis of temporal evolution characteristics of thermodynamical structure and stormrelated environment in different positions of a squall line that occurred in BeijingTianjinHebei Region on 30-31 July 2013. The study is based on rapidrefresh 4DVar (RR4DVar) analysis system using 6 CINRAD radar observations combined with regional 700 automatic weather stations. The results indicate that the storm develops under favorable weather conditions. When the squall line forms there is strong warm and wet air convergence, rising motion, strong middlelevel vertical wind shear, and positive and negative vorticity approximately balanced in front of middle and southern sections, which contribute to the organization and development of middle and southern sections. In comparison, the front of northern section is under the influence of weak middlelevel vertical wind shear and divergence and sinking movement, which is not favorable for the development of northern section. As the squall line spreads down from the mountain, the front parts of middle and southern sections are under the control of relatively strong middlelevel vertical wind shear, thus strong warm and wet air convergence result in strong dynamic lifting. Also the warm and wet flow is stably conveyed to the ascending area in storm. All these are beneficial to newborn convective cells and high organization of middle and southern sections. During the squall line evolves into the bow echo, strong middlelevel vertical wind shear, outflow boundary and cold pool dominate the middle section. Moreover, the terrain forcing effect is extremely significant. These all are conductive to the convergence and aloft motion of lowlevel warm and wet airflow southerly in front of the bow echo, which are the main causes for the quick enhancement of bow echo and stable maintenance during the squall line down the mountain.