The invention discloses a high strength corrosion resistant rare earth cast magnesium alloy and a preparation method thereof, which is characterized in that: the magnesium alloy comprises the following components according to the weight percentage: 1.8 to 4.0 percent of Nd, 1.0 to 2.0 percent of Gd, 0.2 to 0.8 percent of Zn, 0.4 to 1.0 percent of Zr, 0.0 to 0.8 of Ca, not more than 0.02 percent of total content of impurity elements of Fe, Cu and Ni, and the balance of Mg complemented to 100 percent; and the preparation method comprises the following steps of: melting pure magnesium, adding Mg-Nd intermediate alloy, Mg-Gd intermediate alloy, Mg-Ca intermediate alloy and commercial pure Zn in sequence, heating up to 760 DEG C to 785 DEG C after the mixture is totally melted, adding Mg-Zr intermediate alloy, stirring fully and evenly after the Mg-Zr intermediate alloy is melted, standing for 10 to 20 minutes, removing dross, and then casting ingots or casting the cast products at the temperature of 720 DEG C to 750 DEG C. The cast magnesium alloy has the advantages of controllable melting process, appropriate cost, excellent mechanical property, corrosivity, good casting process property and the like; and the cast magnesium alloy can be used to manufacture various non-die casting magnesium alloy parts by adopting the processes of sand mold casting, low-pressure casting, counter-pressure casting or squeezing casting, etc.