The outputs of the η mesoscale numerical simulation, the observational data, the infrared cloud image from GMS and the TBB data are employed to study the structure and formation of a low vortex on the shear line and the sources of the cold air. The results show that the low vortex in this process belongs to the meso-α scale, with 7-hour duration. In the mature phase, the center of the positive vorticity is coincident with that of the vortex, and the 700hPa low-level jet and the strong convergent center located in its southeast quadrant. The vortex circulation is made up of three currents, with the super-strong divergence column, the vertical vorticity column, and the deep ascending motion region above. The deep ascending motion is coupled with the strong divergence. The centers of the strong convergence (divergence) emerged at the positions of the convergence (divergence) of southerly and northerly, accompanying with the strong vertical shear of meridional wind. In addition, there is a negative vorticity center over the low-level positive vorticity center. It is also found that the maximum vapor convergence located at 700hPa and 550hPa and that the cold air is mainly derived from the high over Yunnan to Burma at the 500hPa.