In recent years, research scholars have conducted in-depth research on the efficient and stable electrocatalysts for oxygen reduction reaction (ORR) and analyzed how to reduce the polymer binders in the cathode of metal–air batteries. Although some research progress has been made, there are still many challenges. On the basis of full consideration of factors such as good electrical conductivity, unique porous structure, improved performance, surface area, and synergistic effects, composite materials (m-Co3O4/3DG) of three-dimensional graphene and mesoporous Co3O4 nanowires are prepared on a Ni foam substrate. Here, a facile solution (NaBH4) reduction method is introduced, resulting in an increase in the oxygen vacancies on the surface of the mesoporous Co3O4 nanowires, thereby improving the electrocatalytic activity of m-Co3O4/3DG ORR. The reduced m-Co3O4/3DG composite exhibits a much positive half-wave potential of 0.84 V (vs. RHE) and onset potential of 0.93 V (vs. RHE), and much higher mass activity of 0.109 A mg−1 at 0.766 V (vs. RHE) compared with m-Co3O4/3DG, m-Co3O4, and 3D graphene electrocatalysts, or even superior to the Pt/C catalyst. In addition, when reduced m-Co3O4/3DG is used as the binder-free cathode of the aluminum–air battery, showing a 422.74 mAh g−1 specific capacity and a 1.53 V open-circuit voltage under the discharge current density of 1.0 mA cm−2, which are more excellent compared with the traditional Pt/C, m-Co3O4, and m-Co3O4/3DG electrodes. The 1D and 3D morphology hierarchically porous feature leads to high-efficiency surface reduction and charge and mass transportation. This research provides a solution reduction method with a simple operation to generate more defect states on electrocatalysts and supplies valuable view into the development of reduced Co3O4/3DG with excellent ORR electrocatalytic performance as binder-free cathodes in aluminum–air battery.