Dear @Xiaogang.Huang,
The 6x6 rigid-body mass matrix generated through the linearization of FAST or OpenFAST should have the following form:
where
m = mass
(x_g,y_g,z_g) = center of mass relative to the platform reference point
(I_11,I_12,…I_33) = inertias relative to the platform reference point.
So, you can calculate the center of mass relative to the platform reference point from the off diagonal entries. Based on knowledge of the center of mass, you can shift the inertias to a different reference point using the parallel axis theorem.
Best regards,