With its extremely high thermal conductivity, hardness and excellent infrared transmission properties, diamond is the most ideal material for infrared windows under extreme conditions. However, since the theoretical infrared transmittance of diamond is only 71%, further development of diamond surface anti-reflection coating has become a key step in the improvement of diamond infrared window. Compared with the traditional infrared anti-reflection coating, Y₂O₃ has lower refractive index, wider anti-reflection band and stable optical properties, which is an ideal diamond infrared anti-reflection coating, but poor mechanical properties make it difficult to prevent external damage in extreme environments. In the current study, the mechanical properties can be changed by changing the phase structure of the anti-reflection membrane itself, but it is difficult to improve the mechanical properties by changing the growth parameters for phase regulation. Rare-earth doping can effectively change the structure of the matrix material and improve its performance. The Y₂O₃ film deposited by the magnetron sputtering method has strong adhesion and high purity of the membrane layer. Moreover, the oxygen-argon ratio can be controlled in the process of preparing the oxide film, which is more conducive to obtaining the oxide film close to the stoichiometric ratio. Therefore, undoped and La-doped Y₂O₃ films were prepared on mono-crystalline silicon and poly-crystalline CVD diamond by magnetron sputtering method. During the RF reaction sputtering, the target atoms of Ar plasma react with the reaction gas O₂, and the Y₂O₃ film is deposited on the substrate surface. By adjusting the RF sputtering power of the doped element La target, the doping content of La element is adjusted. The composition, structure and properties of La-doped Y₂O₃anti-reflection films were studied. X-ray photoelectron spectroscopy (XPS) and graze-incidence X-ray (GIXRD) studies show that metal La interacts with O and exists in Y₂O₃ films in the form of La-O compound. The undoped Y₂O₃ films show cubic (222) columnar crystal orientation, and with the increase of La doping power, the films show monoclinic Y₂O₃ crystal orientation (111). It can be observed by scanning electron microscopy (SEM) that Y₂O₃ films with different La doping power show columnar crystal structure and good crystal quality. Atomic force microscopy (AFM) confirms that La-doped Y₂O₃ films have lower roughness (RMS) values than undoped Y₂O₃ films. In the La-doped Y₂O₃ films, the grain size of the columnar crystals decreases significantly with the increase of La concentration. In the long-wave infrared range of 8-12 μm, the maximum transmittance of La-doped Y₂O₃/Diamond film is 80.3%, which is 19.8% higher than that of CVD diamond film. La-doped Y₂O₃ films with fine particles have higher hardness and elastic modulus. The hardness increases from undoped (12.02±0.37) GPa to (14.14±0.39) GPa, and the elastic modulus increases from (187±14) GPa to (198±7.5) GPa. After La-doped Y₂O₃ film, the grain was refined and the roughness decreased. La-doped Y₂O₃ film was subjected to the maximum transmittance increasing from 67% to 80.3% (LWIR), and the optical performance was significantly improved. In addition, the mechanical properties of the La-doped Y₂O₃ films were improved. The main reason for this phenomenon is mainly attributed to the presence at the grain boundary of Y₂O₃ film after La doping, which hinders the growth of Y₂O₃ grains to play the strengthening of fine crystals and improves the mechanical properties of the film. The results show that compared with the undoped Y₂O₃ films, the La-doped Y₂O₃ films obtain higher hardness through fine crystal strengthening under the condition of keeping higher infrared transmittance, which is conducive to improving the erosion properties of sand and rain erosion.