A mild and environmentally friendly photocatalytic method for C-C bond formation between 1,3-dicarbonyls and styrene derivatives has been developed in a green solvent - ethanol. A series of α-functionalized β-diketones were obtained in moderate to good yields. Based on the results of control experimental and theoretical calculations, the photocatalytic transformation might be accomplished by generating reactive radicals via a single electron transfer process. Moreover, the matching of reduced-state photocatalyst with the radical intermediate is considered to be critical for this conversion.