In order to improve the adaptability of infrared windows in harsh wet environments, Yb2O3-x films were prepared by ion assisted reactive thermal evaporating deposition. The surface morphology, microstructure, composition, optical properties, hardness, residual stress and hydrophobicity of Yb2O3-x films were comprehensively characterized. The effects of the oxygen flow rate on the properties of the Yb2O3-x films had been explored. It is found that the root-mean-square roughness of the films increases as the flow rate increases from 10 sccm to 35 sccm. The crystallization characteristics are closely related to the oxygen flow rate, and the film prepared at 15 sccm exhibits the best crystallinity. X-ray photoelectron spectroscopy (XPS) testing shows that the ratio of Yb to lattice oxygen decreases, while the content of trivalent Yb ions in the film increases with increasing oxygen flow rate. Thin film prepared at lowest oxygen flow rate exhibits highest optical absorption, which is related to oxygen defects in the films. Moreover, films prepared at 15 sccm, 25 sccm and 35 sccm show good transparency in the long-wave infrared band. The hardness of the film prepared at 15 sccm is about 9 GPa. Due to change in density of the thin film, the compressive stress of the film decreases with increase of oxygen flow rate. The hydrophobicity of the film is affected by the surface chemical composition and surface roughness, and the maximum water contact angle is 100°. In a word, the Yb2O3-x films are found to be excellent hydrophobic protective coatings for infrared windows.