Abstract: XKS shear-wave splitting has been widely applied to investigations of anisotropy in both the upper mantle and the lowermost mantle D" layer, and represents a key tool for probing mantle deformation and dynamics. The first part of this paper reviews the basic principles and major approaches of XKS splitting analysis, and summarizes its applications to upper-mantle anisotropy beneath the Chinese mainland, where strong regional variations in deformation are observed, primarily controlled by the subduction of the Indian plate and the western Pacific plate. We further discuss methods, applications, and challenges in using XKS phases to probe D" anisotropy, highlighting that complex and strong anisotropic structures are commonly observed beneath remnant slab regions and large low-shear-velocity provinces, while effective separation of upper-mantle contributions and mitigation of wavefield scattering and interference remain major difficulties. Over the past decade, with the rapid growth of seismic datasets and advances in inversion techniques, XKS-based three-dimensional anisotropic imaging of the upper mantle has become increasingly mature, significantly improving depth resolution of anisotropic structures. The second part of this paper introduces the theoretical framework and development of this imaging approach and summarizes recent results. This method is capable of delineating three-dimensional upper-mantle anisotropy and effectively identifying deep structures such as slab geometries, mantle upwellings, and layered anisotropy, although the imaging quality strongly depends on data coverage and observation density. Finally, we discuss future perspectives of XKS studies in mantle anisotropy research. The field is evolving from traditional parameter-based analyses toward full three-dimensional imaging. However, three-dimensional anisotropic imaging of both the upper mantle and the D" layer beneath the Chinese mainland remains limited. Future efforts should integrate multiple anisotropic constraints, promote multi-layer mantle coupling studies, and incorporate artificial intelligence techniques to improve the accuracy of shear-wave splitting analysis, thereby providing a foundation for constructing high-resolution, high-precision three-dimensional anisotropic models of the mantle beneath China.