A dual-function beam separation grating with double structure periodic ridges is proposed. The parameters of the grating structure and characteristics of the incident light are all optimized by the finite-element method and rigorous coupled-wave analysis, so the grating can display the polarization-selective characteristic. At the parameter values that will be discussed in Sec. 2, the diffraction efficiency of the zeroth order, first order, and second order for transverse electric (TE) polarization is 32.30%, 32.34% and 32.30%, respectively, and the uniformity is calculated to be 99.88%. For transverse magnetic (TM) polarization, the reflectivity of the zeroth order and second order is 47.23% and 47.24%, respectively, and the uniformity of two ports is 99.97%. Compared with most previously reported polarization-selective beam splitters that achieve single-port output for TE polarization and dual-port output for TM polarization, the grating realizes TE-three/TM-two output with a decent diffraction efficiency uniformity.