In this research, a controlling mechanism of particle size on the wettability of Ag2O particles is reported, and an interesting floatable Ag2O photocatalyst with superhydrophobicity and superoleophilicity is prepared based on this mechanism. Stable superhydrophobic and superhydrophilic Ag2O without a low-surface-energy modifier can be obtained only by adjusting its particle size, and its wettability can switch mutually by changing the particle size. The wettability of Ag2O converts from superhydrophilic to hydrophobic when the average particle size is more than 1.08 μm. The operation parameters of the Ag2O crystallization process significantly influence the wettability of the Ag2O particles. The obtained superhydrophobic Ag2O floated on the water surface, and exhibited excellent photodegradation performance with various floating oils. This attractive superhydrophobic Ag2O photocatalyst is promising for practical applications, and provides a strategy for the development of functional photocatalysts and superhydrophobic materials.