The invention relates to a preparation method for a Mg-Ga intermediate alloy, belonging to the field of industrial magnesium alloys. The preparation method comprises the following steps: heating magnesium powder under the protection of argon to fully remove water vapor on the surface of the magnesium powder; placing metallic Ga in a plastic vessel, heating metallic Ga with hot water so as to obtain liquid metallic Ga and carrying out ultrasonic treatment after complete dissolving; adding the magnesium powder into the liquid metallic Ga and carrying out mechanical stirring; placing a mixture obtained in the previous step in an environment with a temperature of -20 to 20 DEG C to allow metallic Ga to be completely solidified and carrying out cold isostatic pressing at low temperature so as to obtain a blank material; putting the blank material into a ceramic container, sealing the ceramic container, putting the ceramic container into a muffle furnace and carrying out heat treatment in stages; taking out the blank material and directly carrying out canned extrusion; and carrying out heat treatment under the protection of argon after extrusion and then carrying out cooling to room temperature in the furnace. The intermediate alloy prepared by using the method has a uniform structure, can effectively improve the yield of elemental Ga, reduces environmental pollution, increases the varieties of Ga-containing alloys and broadens the application field of Ga-containing alloys.