The invention discloses a casting magnesium alloy as well as a preparation method and an application thereof, wherein the preparation method comprises the following steps: (1) mixing and smelting magnesium, aluminum and zinc to obtain a mixture M1; (2) adding rhenium and zirconium into the mixture M1 and smelting to obtain a mixture M2; and (3) adding silver into the mixture M2 to smelt to obtain the casting magnesium alloy, wherein based on 100 parts by weight of magnesium, the use level of aluminum is 1-10 parts by weight, the use level of zinc is 1-5 parts by weight, the use level of rhenium is 0.1-2 parts by weight, the use level of zirconium is 0.2-3 parts by weight and the use level of silver is 0.05-0.5 part of by weight. According to the design, by mixing magnesium, aluminum, zinc, rhenium, zirconium and silver in a certain proportion and smelting, the magnesium alloy prepared by the method is good in casting performance under a high-temperature condition, so that the range of application of the prepared magnesium alloy is greatly increased and the service life is prolonged.