In this paper, the composite micro-nano structure surface of Al₂O₃-Si with a low ratio of depth to width and full duty ratio was prepared on the monocrystalline silicon substrate by using the self-assembly mask etch technology as well as the conformal growth feature of atomic layer deposition technology. The results of spectral reflectance tests show that when the bottom attains the full duty ratio and the ratio of depth to width is close to 1∶1, the average reflectance of the composite micro-nano structure surface in the frequency band of 3--5 μm is less than 3.5% at an incident angle of 8°. The results of nano-indentation tests demonstrate that the elastic recovery rate of the composite micro-nano structure surface of Al₂O₃-Si is 10.14% higher than that of single silicon substrates, which proves that the deposited alumina film can improve the mechanical properties of the anti-reflection micro-nano structure.