The invention relates to the technical field of magnesium alloy preparation, in particular to a nano-silicon-carbide-modified Mg-Al-Zn magnesium alloy material for casting automobile parts and a preparation method of the material. The material is a high-heat-resistance Mg-Al-Zn alloy and is doped with Si, Bi and nanoscale Nb elements, the high-performance composite alloy material is obtained, the utilization rate of the elements is higher, besides, a polyvinylpyrrolidone anhydrous ethanol solution is used as a medium, nano silicon carbide is dispersed in the medium, then the mixture is mixed with Mg powder for grinding, nano silicon carbide is evenly coated with and dispersed in the Mg powder, mixing with other metal elements in the subsequent smelting process is more sufficient, the modification effect is better than that of a direct doping method, a prepared magnesium alloy is fine in structure, high in hardness, light in mass, corrosion-resistant, wear-resistant, heat-resistant and oxidation-resistant, and the processability and use performance of the automobile parts prepared from the material are improved.