The “inverse rule” Fourier expansion is used to enhance the convergence of perpendicularly crossed surface-relief binary-period gratings. The reflection-transmission-coefficient-matrix (RTCM) algorithm used in single-period gratings is extended to perpendicularly crossed binary-period gratings. The symmetry and diagonal period are well used, keeping certain computational accuracy and steady, to increase the speed of computation to tens of times, so that the calculation of such problem can be done by PC instead of large computers.