The invention provides high-strength, high-ductility and heat-resisting Mg-Gd-Er alloy applicable to low-pressure casting and a preparation method for the same. The alloy is constituted of chemical ingredients including, by mass, 3.0%-7.0% of RE, 1.2%-4.2% of Zn, 0.5%-1.2% of Al, 0.1%-0.3% of Mn, 0.01%%-0.08% of M as well as Mg, wherein M comprises one or two elements between Ti and B. The preparation method comprises the steps that (1) the ingredients of the Mg-Gd-Er alloy are prepared; (2) industrial pure magnesium ingots are melted; (3) the temperature is increased to 700 DEG C, and industrial pure Zn and intermediate alloy of Mg-Gd, Mg-Er and Mg-Mn are melted; (4) the temperature is increased to 730 DEG C, and industrial pure aluminum ingots and intermediate alloy of Al-Ti, Al-Ti-B andAl-B are melted, so a Mg alloy melt is obtained; (5) low-pressure casting is carried out; and (6) secondary solid solution processing and artificial ageing processing are carried out. The high-strength, high-ductility and heat-resisting Mg-Gd-Er alloy applicable to low-pressure casting provided by the invention has the advantages that tensile strength under a room temperature reaches 305Mpa and aductility rate is 18% after the low-pressure casting, secondary solid solution processing and artificial ageing processing; under high-temperature tension at 200 DEG C, the tensile strength reaches 214 Mpa and the ductility rate is 22%; and the alloy satisfies high-end demands for light-weight development in industries such as aerospace and automobiles.