Western Yunnan is located within a gigantic Tethys orogenic zone between Gondwana and Laurasia. Magmatic activity records of the associated Wilson orogenic cycle show that the causes of plate spreading in the region might have been related to sub-mantle plume. Tectonics, geophysics, sedimentary strata, tectonic evolution of the lithosphere and other research results indicate that there is geological evidence for mantle plume magmatic activities in West Yunnan. Tomography also supports the idea that there is a subducting slab near the Red River fault and an upwelling mantle plume in West Yunnan. Here our research presents seismic evidence for upwelling mantle plume, which is a main cause leading to subducting slab in West Yunnan. The analysis is based on compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations in the Sichuan-Yunnan seismic network, and made a study on 410-km and 660-km discontinuities, as well as on three sections of CCP stacking.