The invention discloses a preparation method of a porous magnesium alloy, comprising the following steps: mixing MgH2 powder with a polyoxyethylene-polyoxypropylene acetone solution to prepare a gelatinoid; preserving heat for at least 40min at temperature of (-2) to (-5) DEG C in an injection container, wherein the diameter of an injection needle hole is 100-500 micrometers; installing the injection container on equipment of a 3D automatic printer; controlling the 3D automatic printer to print the gelatinoid in the injection container to a glass substrate at normal temperature according to a preset pattern, so as to obtain a porous magnesium alloy green body; putting into a vacuum furnace for heating after standing at room temperature, wherein temperature is raised to 180-200 DEG C at the rate of 1-3 DEG C/min to preserve temperature for 30-60min, and the temperature is raised to 400-500 DEG C at the rate of 1-3 DEG C/min to preserve temperature for 60-120min; and cooling together with the furnace, thus obtaining the required porous magnesium alloy. The method can effectively avoid the problem of magnesium alloy oxidization and combustion needing to the solved by conventional methods, and is simple and easy to control and suitable for industrial production.