A facile metal-free synthesis of multi-substituted pyrimidines from readily available amidines and α,β-unsaturated ketones is reported. The synthesis involved a [3 + 3] annulation to form a dihydropyrimidine intermediate, which was converted to pyrimidine through visible-light-enabled photo-oxidation rather than the usual transition-metal-catalyzed dehydrogenation. The mechanism of the photo-oxidation was studied. This work has provided an alternative approach to pyrimidines with the advantages of easy operation, and mild and green conditions with a broad scope of substrates, circumventing the dependence on transition-metal catalysts and strong bases.