A two-dimensional polarization-independent subwavelength grating structure suitable for long wave infrared band have been proposed. Symmetric two-dimensional structure is used to eliminate polarization dependence of one-dimensional grating and increase luminous flux. The evaluation function of grating optimization was proposed， and then the optimization of grating structure parameters was transformed into a path optimization problem. Finally， the ant colony algorithm was used to solve the optimal grating structure parameters. Simulation results show that the proposed two-dimensional multilayer structure has the advantages of polarization independence， narrow grating period， narrow spectral bandwidth and low side band in 10～13 μm band. The convergence speed of the proposed optimization design method is fast. The transmittance of the polarization direction of transverse electric （TE） and transverse magnetic （TM） reaches 70%， and the spectral bandwidth is 260 nm.