The genesis mechanism and evolution of a plateau shear line are analyzed. This process involved the weakening of an eastward moving plateau vortex and the formation of a plateau shear line, which moved southeastward later under the favorable background circulation and caused heavy rain in Sichuan and Chongqing. The Okubo-Weiss (OW) parameter (V_OW) is introduced first. The OW parameter is frequently used in tropical cyclone study to measure the relative amount of rotation and deformation in vortex and shear flow, and has a good potential for identifying the location of plateau shear line formation and development. The results show that, at 500 hPa the V_OW value changed from positive to negative and V_OW negative belt had a good potential for identifying the location of plateau shear line formation at the genesis stage of plateau shear line. The intensity of negative V_OW value and plateau shear line had a good correlation. The plateau shear line was dominated by negative V_OW centers, but positive center also existed, indicating that cyclonic vorticity also occurred in the shear line. This plateau shear line was dominated by stretching deformation and located along the stretching axis orientation in this case. The dynamic mechanism of plateau vortex weakening and plateau shear line formation is then further analyzed by using the vorticity equation and the total deformation equation. The results show that the weakening and disappearance of the plateau vortex were mainly affected by the divergence term. The temporal evolution analysis indicates that this system transformed from plateau vortex with strong cyclonic vorticity to plateau shear line with strong convergence. The twisting term in the total deformation equation made the largest contribution to the formation of the plateau shear line, followed by the horizontal pressure gradient term, and the divergence term made the minimum contribution. When the plateau shear line was dominated by stretching deformation, it restrained the development of plateau vortex. The shear line could be the background flow field that affected the vortex development.