Graphene films which are fabricated by simple dispensing method without surface modification exhibit reversible switching between superhydrophobicity and superhydrophilicity under alternative UV illumination and dark storage. Careful study of the mechanisms is conducted in view of surface geometry, microstructure, and chemical compositions, and focused on chemical compositions. Photo-induced chemisorption of hydrophilic radicals and water molecules on graphene is found to be responsible for the transition from original hydrophobicity to hydrophilicity, even superhydrophilicity, while replacement of water molecules by oxygen during storage in the dark environment induces the hydrophobicity recovery of graphene films from hydrophilic state. Appearance and disappearance of hydroxyl radicals are closely related to the lattice structure of graphene and energy supply. This wettability conversion is fully reversible. The outcomes not only reveal micro-processes involved in wettability conversion of graphene but also help to purposely design and produce devices from carbon materials with tailor-made tunable surface wettability according to our actual need.