This study used multi-scale transform and positive pressure perturbation method to simplify the barotropic quasi-geostrophic vorticity equation. For a class of cosine type shear, analyzes the influence of shear westerly on Rossby solitary wave that has nonlinear Shrӧdinger envelope soliton characteristics, and studied the topology of the westerly with nonlinear Shrӧdinger envelope soliton wave pattern. The results show that: (1) The key to the formation of cut-off low pressure is the bifurcation of westerly shear. The shear is not only the source of the envelope Rossby solitary wave, but also leads to the change of the structural characteristics of the westerly flow field. The essence of difference between different flow patterns is the structural change that is caused by bifurcation. It indicates that there exists a critical value for shear, which changes the topology of westerly. The topology of westerly has no singular points, only a degenerate central structure, and the corresponding westerly is a zonal flow, when the shear is less than this critical value. There are singular points, and vortices are generated in the westerly, when the shear is greater than this critical value. (2) For the vortices pattern of westerly, it exhibits different flow patterns, if the location of the singular points varies. The westerly presents an Ω shaped blocking flow field, when the singularity point appears only in the northern part of the system. The westerly presents an inverted Ω shaped cut-off flow field, when the singularity point appears only in the southern part of the system. And the westerly presents a dipole blocking flow field, when singular points appear in both the north and south. (3) There are cut-off low-pressure in both the inverted Ω type field and the dipole blocking flow field. It is a Northeast Cold Vortex, if the cut-off low-pressure forms near the Ural Mountains and moves downstream to the northeast region of China. Based on the blocking situation as the source of the Northeast Cold Vortex, the formation mechanism of the Northeast Cold Vortex is discussed with both the board view (westerly shear) and local characteristics (Rossby solitary wave) considered, and thus deepens the understanding of the formation of the Northeast Cold Vortex.