The invention discloses a low-density flame-retardant polypropylene nano-composite material. The low-density flame-retardant polypropylene nano-composite material is characterized by comprising 40 to 60wt% of polypropylene, 20 to 40wt% of nano-hydrotalcite-like compound, 10 to 20wt% of nano-talcum powder and 5 to 10wt% of a processing assistant. The invention also discloses a preparation method of the low-density flame-retardant polypropylene nano-composite material. The low-density flame-retardant polypropylene nano-composite material has the advantages that synergism of the hydrotalcite-like compound and the nano-talcum powder is superior to simple superposition of effects of the hydrotalcite-like compound and the nano-talcum powder; flame retardation is improved; a total addition ratio of the hydrotalcite-like compound and the nano-talcum powder is reduced; and high strength, light weight and flame retardance of an automobile component are obtained.