This paper addresses the coating requirements for 2.7 μm acousto-optic Q-switches and details the design, fabrication process, and performance testing of 2.7 μm anti-reflective coatings. The substrate material for the 2.7 μm anti-reflective coating is lithium niobate crystal, with hafnium oxide selected as the high-refractive-index material and ytterbium fluoride as the low-refractive-index material. A four-layer narrowband high-transmittance film design was employed in this study. The resulting samples exhibited a transmittance of 98.6% at a wavelength of 2.7 μm. The coated samples maintained their stable performance after 48 h of high- and low-temperature experiments and 24 h of immersion in common solvents such as acetone and anhydrous ethanol.