Atomically thin transition metal dichalcogenides (TMDCs) are intriguing semiconductors for photonics and optoelectronics, and therefore enhancing their photoluminescence (PL) efficiency is crucial for these applications. Many efforts have been contributed to enhancing the PL performance of monolayer TMDCs, yet the complexity between the microstructure and the PL efficiency has hindered the manipulation of their PL properties. Here we demonstrate that the PL intensity of the monolayer TMDC can be enhanced by nearly one order of magnitude with a ∼20% narrower spectral linewidth after a pre-activation plateau using laser irradiation in ambient environment. Combined experimental and theoretical studies reveal that low-power laser irradiation generates many sulfur vacancy clusters, which are subsequently filled up by oxygen, and the lattice substitutional oxygen clusters induce the dramatic PL enhancement of monolayer WS₂. Such PL enhancement phenomenon is found to be universal for other monolayer TMDCs, and thus would benefit their versatile optical applications.