In view of the problems that the existing sub-wavelength anti-reflection grating structure parameters have high requirements on processing technology and are difficult to manufacture, a square cylindrical two-dimensional anti-reflection grating structure is designed. Based on the equivalent medium theory and thin-film anti-reflection theory, the structural parameters of the subwavelength anti-reflection grating are simulated and analyzed. The results show that the average transmittance of the far-infrared band at 20~24 μm can reach 98%, and the transmittance at the central band of 21.8 μm reaches 98%. 99%. The femtosecond laser direct writing method was used for experimental preparation, and the measured average transmittance reached 77%, and the central band transmittance reached 77%, which was similar to the simulation result. It is verified by experiments that the process tolerance of the subwavelength grating structure designed in this paper is better than that of the existing grating structure, and the requirements for experimental processing accuracy are lower, which provides a new way of thinking for the design and processing of other similar grating structures.