Dear @Huajian.Xiao,
The stiffness matrices reported in the OC3-Hywind specifications report, are reported relative to the OpenFAST origin, which is the intersection of the tower centerline and SWL. And my comments about K_COG were also made in reference to this origin.
You can always convert the matrices from one coordinate system to another (such as the full-system COG), e.g., as discussed in the following forum topic: OC3-Hywind RAOs.
Best regards,