A low-density high-noise-reducing polypropylene nano-composite material and a preparation method thereof are disclosed. The low-density high-noise-reducing polypropylene nano-composite material comprises the following raw materials in percentage by weight: 86-94% of polypropylene, 3-9% of nano-filler, 1-3% of dispersant, 0.1-1% of antioxidant and 0.1-2% of other additives, wherein the nano-filleris a mixture of organic modified montmorillonite and carbon nanotubes, and the weight ratio of the nano-filler to the carbon nanotubes is 2:1. The preparation method comprises the follow steps: weighing raw materials according to a weight ratio, uniformly mixing polypropylene, the dispersant, the antioxidant and other additives in a high-speed mixer, adding the mixture into a twin-screw extruder,adding nano-fillers in the second half of the extruder through side feeding, and granulating by melting extrusion and cooling. As that organic modified montmorillonite and the carbon nanotube are added, the polypropylene composite material has excellent noise reduction performance and low density at the same time, and can be widely applied in the field of automobile interior and exterior ornamentsand the like, thereby meeting the application requirements of automobile lightweight.