Abstract: This paper describes an improved non-overlapping and non-conformal volume-surface integral equation domain decomposition method (VSIE-DDM) for the complex finite frequency selection surface (FSS) structures. To efficient electromagnetic analysis of this complex FSS structure, based on the recently developed domain decomposition method (DDM), we first developed different partitioning methods, each subdomain must not be identical and can be arbitrarily shaped. It makes the VSIE-DDM more flexible. Then, due to the fine cells of the FSS and the thin dielectric substrate, the mesh is denser, so a matrix with a larger dimension is obtained. In order to calculate the inverse of the self-coupling matrix of the subdomains of larger dimensions more efficiently, an inner-outer iterative technique is used. Subdomains with larger electric size can be employed, which makes the DDM obtain better convergence and further improve the simulation efficiency. We show several numerical examples to verify the computational performance of the algorithm used in this paper.