The invention relates to heatproof magnesium stannum silver alloy and a preparation method thereof, which belongs to metal structural materials and the field of preparation technology. The ingredient of the magnesium stannum silver alloy is composed of four elements of magnesium, stannum, silver and manganese or five elements of magnesium, stannum, silver, manganese and zinc. The mass percentage of the content of each ingredient is stannum (Sn) of 6% to 7%, silver (Ag) of 1% to 2%, manganese (Mn) of 0.2 % to 0.5%, zinc (Zn) of 0% to 1.0% and magnesium of the rest. The preparation method comprises steps of sequentially placing all ingredients in a crucible, heating, stirring, standing to form alloy liquid, pouring into cold molds to form blanks of the magnesium stannum silver alloy, heating in the air, quenching in cold water to obtain homogenized magnesium stannum silver alloy and finally performing aging hot treatment in the air and quenching in cold water to achieve the heatproof magnesium stannum silver alloy. The magnesium stannum silver alloy is small in content of elements and simple in smelting technology. Under 200 DEG C, the aging peak value can reach more than 84 HV. Aging of 700 hours, hardness can keep above 77 HV. Thus, high temperature tissues are stable, and commercial potentiality is provided.