A one-step, two-component visible light-mediated CoCl2·6H2O-catalyzed oxidative acylation of alkenes by aldehydes to synthesize α,β-epoxy ketone has been achieved in water at room temperature. The photocatalytic activity of Co(II) presented a remarkable achievement for synthesis of α,β-epoxy ketones from aldehydes and olefins, with a wide substrate compatibility including aromatic, heteroaromatic and aliphatic aldehydes, styrenes with both electron-donating and withdrawing groups, α-substituted styrenes, stilbene, acrylates, and even the challenging unactivated aliphatic alkenes. Mechanistic studies including radical trapping experiments, intermediate detection by GCMS, Hammett analysis, and DFT studies unveil the nature of the photocatalytic pathway.