To investigate the properties of Y2O3 film with different crystal structures and the influence rules on the an-ti-reflection performance of diamond. Y2O3 films with cubic and monoclinic crystal structures were prepared by reactive magnetron sputtering on the diamond window by changing oxygen-argon ratio. Then, the properties and anti-reflection performance of two films were studied systematically. Y2O3 films with cubic structure were obtained at low oxygen argon ratio and monoclinic structure were obtained at high oxygen argon ratio. Surface roughness of the two films was 2.57 nm and 1.07 nm, respectively. Both crystal structures showed valence states according with atomic ratio of Y2O3. Hardness of two Y2O3 thin films with cubic and monoclinic structures was 17.4 GPa and 12.6 GPa, respectively. Elasticity modulus was 248.1 GPa and 214.6 GPa, respectively. The maximum transmittance of diamond film was 89.1%, and 24.5% of transmittance was obtained after the Y2O3 film with cubic structure was deposited on both sides of diamond. The maximum transmittance of diamond film was 90.4%, and 25.4% of transmittance was obtained after Y2O3 film with monoclinic structure was deposited on diamond. Thermodynamically stable cubic Y2O3 films and metastable monoclinic Y2O3 films can be obtained by controlling oxygen argon ratio. The valence states of O and Y in Y2O3 films with cubic and monoclinic structure are in accordance with their stoichiometric ratios. Y2O3 thin films with cubic structure exhibit higher hardness and elastic modulus. Both structures show good anti-reflection effect on diamond window. Compared with the monoclinic Y2O3 structure, the anti-reflection effect of Y2O3 thin film is better, which is related to the lower refractive index, and the optimal anti-reflection value of Y2O3 thin film with monoclinic structure moves towards the direction of low wavelength compared with the films with cubic structure.