On the basis of synoptic circulation pattern analyses of the torrential rain occurring in Beijing on July 21,2012, the thermal and dynamical structure of the convergence line and the mesoscale vortex above it was analyzed by using output data of the high-resolution Weather Research and Forecasting (WRF) model. A convergence line formed between the strong, cold air from the northwest and northeast and the warm, moist air from southwest and southeast. The latter was lifted by mid-low-level invasion of the northern cold air, which triggered convective instability. The mesoscale vortex was generated and appeared continuously along the persistent convergence shear line in the low-level troposphere, which is similar to the influence of the train effect in torrential rain. The mesoscale vortices were generated at the left of low-level jet, which was located near mountain, and became the direct reacting system for the torrential rain. The precipitation was amplified with the enhancement of the vortices. This vortex formation was related to the low-level convergence line, the low-level jet, and the terrain. The vertical motion caused by the low-level convergence was enhanced by orographic lift, which is attributed to the persistence of the heavy rain. The lifted air appeared in upward and downward branches when contacting the western air flow at middle levels, and a series of the cyclonic mesoscale secondary circulation at lower levels was triggered when the downward branch mingled with the upward air current at those levels. An updraft was persistent above Fangshan district was the main cause for the torrential rain in that location.