This research is to shown an robust electromagnetic method suitable for multiscale simulation domain, especially for LCoS SLM. The data in Fig 4 are the simulated diffraction efficiency of a fictitious 2D LCoS SLM using typical FEM method, FEM combined with scattering matrix method, and the proposed method, which is using scatter matrix method to combine FEM and RCWA simulation. The data in Fig.5 contain both simulation and experimental results of an actual LCoS SLM device. The simulated data is obtained with FEM method that combines with the transferred matrix methods in comparison to the proposed method. The experimental data is obtained with LCoS SLM that displayed a 6 pixel blazed grating with pixel pitch 8um.