Dear @Li.Kang,
Are you seeking the mass and stiffness matrix for the full wind turbine? Which structural DOFs do you want enabled?
ElastoDyn includes geometric nonlinear terms, so, the mass matrix depends on the system displacements and the full stiffness matrix is not formed within the time-domain solution. That said, the linearization functionality of OpenFAST allows you to linearize the model at a given operating point so as to enable a linearized representation of the system about a given condition. The linearized system is expressed in first-order form, meaning the mass matrix is not directly output, rather, the mass and stiffness show up as -M^-1K in the state matrix A. However, you can often infer the mass and stiffness through other linearized matrices, as has been discussed in other forum topics, e.g.: OpenFast 2nd order Linearization.
Best regards,