Due to the unique character of methyl viologen (MV2+), namely, the electron oscillation that may occur in the process of MV2+ reduction and oxidation, by means of MV2+ addition, reacting system inert-atmosphere and electron donor addition, based on the peculiar contribution of MV2+ with an electron oscillation characters, the stabilities and hydrogen evolution of the Co3O4-g-C3N4 photocatalyst were greatly improved. Charge separation and electron transfer were more efficient under the condition of MV2+ addition, as supported by fluorescence studies. The photoreduction of MV2+ by Eosin Y (EY2-) can form a persistent methyl viologen radical cation (MV center dot+). MV2+, as an excellent electron relay, greatly facilitated electron transfer from the excited state (3)(EY2-)* to the conduction band of Co3O4-g-C3N4. Accordingly, the excited state (3)(EY2-)* was oxidized by MV2+; this process effectively promoted the circulation regeneration of EY2-, suppressed the generation of unstable intermediate EY center dot 3-, and reduced the decomposition of EY.