Abstract: To address the issue of considerable interfacial reflection loss of fused silica optical windows over a broad spectral range, a broadband antireflection method based on double-sided biomimetic moth-eye subwavelength micro-nanostructures was proposed. A double-sided hexagonal periodic cylindrical structure model was established based on effective medium theory and the finite element method. The effects of structural height on the equivalent refractive index distribution, electric field distribution, and broadband transmittance were analyzed. Cylindrical micro-nanostructures with heights of approximately 200 nm and 350 nm were fabricated on both sides of fused silica substrates using displacement Talbot lithography combined with inductively coupled plasma etching. Both simulation and experimental results demonstrate that the double-sided subwavelength structures can form a graded refractive index layer at the air-silica interface, by which Fresnel reflection is effectively reduced and broadband transmittance is enhanced. Specifically, a transmittance exceeding 95% was achieved in the visible band of 500 nm~700 nm for the 200 nm-high structures, and a transmittance exceeding 95% was obtained in the near-infrared band of 800 nm~1300 nm for the 350 nm-high structures. It is revealed that the structural height affects both the thickness of the refractive index gradient layer and the light-field modulation capability, by which antireflection optimization for different spectral bands can be realized. This work provides a theoretical basis and a processing reference for the design of broadband fused silica optical windows.