The oxygen adsorption can drastically alter the electronic properties of the two-dimensional (2D) materials, which is usually difficult to be removed. In this work, we report the ultraviolet (UV) illumination induced desorption of the O₂ molecules from the monolayer MoS₂ surface by using the atmosphere dependent transport measurement, Kelvin probe microscopy, photoluminescence spectroscopy and x-ray photoelectron spectroscopy. Obvious increasing of the conductivity, rising of the Fermi level, and red shift of the photoluminescence peaks of the MoS₂ were observed after the UV illumination in vacuum, indicating the elimination of the depletion effect from the oxygen adsorption. Such parameter changes can be reversibly recovered by the subsequent O₂ exposure. Furthermore, obvious decreasing of the oxygen concentration after the UV illumination was also observed by x-ray photoelectron spectroscopy. Thus the UV induced O₂ photodesorption effect is evidenced. The photo-excited charge transfer mechanism is proposed to account for the photodesorption effect.These results provide a nondestructive way to clean the MoS₂ surface and manipulate the performance of the MoS₂ based devices.