The invention discloses a preparation method of an air cathode of a magnesium seawater battery. The preparation method comprises the following steps: taking the prepared catalytic layer and taking foamed nickel as a current collecting layer; preparing a paste substance for diffusion; taking carbon black, polytetrafluoroethylene emulsion and a pore forming agent; putting the carbon black, the polytetrafluoroethylene emulsion and the pore forming agent into ethanol serving as a dispersing agent, and stirring to form paste; removing the ethanol to obtain the paste substance for diffusion; injecting the paste substance for diffusion into a cavity in foamed nickel serving as the current collecting layer; superposing the current collecting layer injected with the paste substance for diffusion and the catalytic layer to form a double-layer integrated structure; and decomposing the pore forming agent in the double-layer integrated structure by a pore forming method to finish pore forming and obtain the air cathode. The preparation method of the air cathode of the magnesium seawater battery has the following advantages: the process is simple, the cost is low, and the prepared air cathode is low in failure rate under long-time seawater erosion and high in discharging current density, so that the electric property of the magnesium seawater battery can be improved.