The invention relates to the technical field of preparation of magnesium alloy, in particular to a nanoscale molybdenum disilicide modified Mg-Al-Si based magnesium alloy material for casting automobile parts and a preparation method thereof. The material is Mg-Al-Si based high-heat-resistance alloy and doped with elements such as Ca, Pr and nanoscale zirconium powder, thereby having a more excellent comprehensive mechanical property. Besides, a polyvinylpyrrolidone absolute ethyl alcohol solution is used as a medium, nanoscale molybdenum disilicide is scattered in the solution, and the mixture and Mg powder are mixed and ground, so that the nanoscale molybdenum disilicide is evenly wrapped and scattered in the Mg powder and is mixed with other metal elements more sufficiently in the later smelting process. The mode has a better modification effect compared with the direct blending mode, and the prepared magnesium alloy is compact in texture, excellent in high temperature resistance, high in intensity and resistant to both abrasion and corrosion; the machinability and usability of automobile parts prepared from the material are both improved.