This paper reports the preparation of α-MnO2 NWs/graphene composites as the cathode catalyst for magnesium-air fuel cell and its excellent electrochemistry performance. The composites are synthesized by self-assembly of α-MnO2 nan α-MnO2 NWs/graphene was synthesized and studied in Mg-air fuel cell. α-MnO2 NWs/graphene was synthesized and studied in Mg-air fuel cell. owires (NWs) on the surface of graphene via a simple hydrothermal method. The α-MnO2 NWs/graphene composites showed a higher electrochemical activity than the commercial MnO2. The oxygen reduction peak of the α-MnO2 NWs/graphene composites catalyst is tested in a 0.1M KOH solution at −0.252V, which is more positive than the commercial MnO2 (−0.287V). The ORR limit current density for 28% α-MnO2 NWs/graphene composite is approximately 2.74mA/cm2, which is similar to that of the 20% Pt/C(2.79mA/cm2) in the same conditions. Based on the Koutecky–Levich plot, the ORR mechanism of the composite involves a one-step, quasi-4-electron pathway. In addition, magnesium-air fuel cell with α-MnO2 NWs/graphene as catalyst possesses higher current density (140mA/cm2) and power density (96mW/cm2) compared to the commercial MnO2. This study proves that the cost-effective α-MnO2 NWs/graphene with higher power generation ability make it possible for the substitute of the noble metals catalyst in the Mg-air fuel cell.