The invention relates to a preparation method for the magnesium-lanthanum praseodymium cerium intermediate alloy, which belongs to the technical field of the fused salt electrolysis metallurgy. Under the conditions that potassium chloride, magnesium chloride and lanthanum praseodymium cerium chloride are taken as electrolyte in an electrolytic furnace, the mass ratio of the magnesium chloride, the potassium chloride and rare earth chloride is controlled as 5: (40-35): (55-60), the electrolysis temperature is 800- 900 DEG C, the current density of a cathode is 10-30A/cm<2>, and the distance between electrodes is 4 to 8 cm, the magnesium chloride and the lanthanum praseodymium cerium chloride are added into during the electrolysis process, the mass ratio of the magnesium chloride and the lanthanum praseodymium cerium chloride is 1: 3 to 5, to obtain the magnesium-lanthanum praseodymium cerium intermediate alloy. The magnesium metal is not used, and the rare earth metal is also not used, but the chloride of rare earth and magnesium is used, to cause rare earth ions and magnesium ions on the cathode to perform the coelectrodeposition to generate the intermediate alloy with melting point close to the eutectic temperature of the are earth and the magnesium. The operability of the method is strong, the process and the equipment are simple, and the products produced in mass are stable and easy for the industrial scale production.