In order to decrease the corrosion rate of magnesium anode and increase the discharge property of magnesium-air battery, a water soluble graphene poly (sodium 4-styrenesulfonate)/reduced graphene oxide and a reduced graphene oxide/Mn3O4 nanometer composite are prepared successfully. They are used to assemble a magnesium-air battery with a poly (sodium 4-styrenesulfonate)/reduced graphene oxide based NaCl electrolyte and a reduced graphene oxide/Mn3O4 nanometer composite as cathode catalyst. The structures and morphologies of poly (sodium 4-styrenesulfonate)/reduced graphene oxide and reduced graphene oxide/Mn3O4 nanometer composite are tested by x-ray diffraction, scanning electron microscope and fourier transform infrared spectrophotometer. The assembled magnesium-air battery with poly (sodium 4-styrenesulfonate)/reduced graphene oxide and reduced graphene oxide/Mn3O4 nanometer composite has been found to possess an energy density of 1620 Wh kg−1 and an anode utilization of 82%, much higher than those achieved with a NaCl solution and a commercial air cathode (1115 Wh kg−1 and 52%).