A polarization-insensitive four-port beam splitter based on the plus-shaped ridge structure is introduced. The ridge of this grating is a plus-shaped resin material. Based on the rigorous coupled-wave analysis and the simulated annealing algorithm, the transmission efficiency of the zeroth order under TE polarization is 1.79%, and the efficiencies of 22.21%/22.19% for the ± first/± second orders are diffracted. Meanwhile, the transmission efficiency of the zeroth order under TM polarization is 0.13%, and the efficiencies of 23.25%/23.23% for the ± first/± second orders are diffracted. In addition, the diffraction efficiency of each order is computed while the wavelength, grating period, and duty cycle are varied around the optimized parameters. The study reveals that the grating has excellent tolerance and stability. Hence, it is inferred that plus-shaped ridge four-port grating exhibits promising potential for advancement in the field of lasers.