Abstract: Based on the direct S-wave seismic records of the DONET seafloor observation network, this paper obtains the orientation of shear wave splitting fast axis and splitting time of Naikai area of Japan by using the cross-correlation method and the smallest eigenvalue minimization method. The results show the fast axis of the anisotropy in the subduction zone is sub-parallel to the strike of the Nankai trough, and the splitting time ranges from 0.1 s to 0.96 s. This indicates that the anisotropy of the Nankai subduction zone originates from the mantle wedge which overlies the subducted Pacific Plate and the subducted Philippine Sea slab. The anisotropy of the mantle wedge is caused by the existence of two-dimensional corner flow that causes the anisotropic mineral crystals to be oriented along the direction perpendicular to the trench. The anisotropy of the subducted Philippine Sea slab is caused by the “fossil anisotropy” and the faults related to the plate bending during the subduction. The widely varying splitting times reflect the inhomogeneous strength and/or thickness of the anisotropic structure beneath the area.