The specific surface area and surface hydrophilicity of metal-organic frameworks can be tailored by changing their central metal ions and organic ligands. In this study, the surface of the metal-organic framework MIL-101(Cr) was modified in an aqueous solution of tetrahydropyran via the radiolytic method. A large number of hydroxyl groups were introduced to the surface of MIL-101(Cr), thereby effectively improving its surface hydrophilicity and substantially increasing its specific surface area without changing its central metal ion and organic ligand. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy demonstrated that the irradiation products of tetrahydropyran were bound to the surface of MIL-101(Cr) through covalent bonding. X-ray diffractometry demonstrated that the crystal structure of the material was retained after irradiation. The specific surface area of MIL-101(Cr) significantly increased, and water contact angle tests indicated that the surface changed from hydrophobic to hydrophilic after modification. These results show that the irradiation reaction of MIL-101(Cr) with tetrahydropyran significantly improves its specific surface area and hydrophilicity. This study provides guidance on the application of the irradiation method to prepare metal-organic frameworks with various properties by simple and rapid modification.