Surface hydroxylation is the basis for material removal in chemical mechanical polishing (CMP) of monocrystalline silicon, diamond, and YAG crystals. Existing studies use experimental observations to investigate surface hydroxylation, but lack in-depth understanding of the hydroxylation process. In this paper, for the first time to the best of our knowledge, we analyze the surface hydroxylation process of YAG crystals in an aqueous solution using first-principle calculations. The presence of surface hydroxylation was verified by X-ray photoelectron spectroscopy (XPS) and thermogravimetric mass spectrometry (TGA-MS) detections. This study complements the existing research on the material removal mechanism of the CMP process of YAG crystals and provides theoretical support for the future improvement of the CMP technology.