Generated from photoelectrocatalytic (PEC) reaction systems, hydroxyl radicals can be applied for the treatment of ballast water contaminated with various microorganisms. However, main cathodic and hydrogen evolution reactions have no effect on the inactivation efficiency, which reduces the overall reaction efficiency. Inactivation efficiency can be improved by inducing the oxygen reduction reactions (ORRs) on a cathode. Oxygen can be kreduced to H2O2 under 2e(-) reactions with ORR catalysts, and OH can be produced by H2O2 under ultraviolet (UV) irradiation, which is beneficial to microorganism deactivation. This work investigated the contributions of ORR in a graphene-like carbon nitride (g-C3N4) to the PEC activities of a ZnWO4 anode. Results indicated that microorganisms were deactivated completely after 18 min under 0.7 V bias potential when ORR was used on the g-C3N4 cathode, and thus approximately one of the total time was conserved. ORR on the g-C(3)N4 cathode plays a key role in PEC inactivation under low voltage potential. The g-C3N4 ORR in seawater involves two electron reactions and generates H2O2, which results in high inactivation efficiency under UV irradiation. This work shows that the search for an efficient 2e(-) reaction ORR catalyst is of considerable importance in improving ballast water treatment.