The invention discloses a method for treating waste polyolefin plastics, which is characterized by comprising the following steps: adding waste polypropylene, ferrocene and sodium azide into a high-pressure kettle, heating to 580-620 DEG C, reacting for 8-24 hours, washing the product with ethanol and water, performing centrifugal separation, and drying to obtain a spongy iron/carbon nanotube composite material, wherein the mass ratio of the waste polypropylene, the ferrocene and the sodium azide is 1:1:(0.25-10). According to the invention, waste polyethylene can be completely decomposed, thus causing no environment pollution; and the yield of the prepared spongy iron/carbon nanotube composite material is up to 90% or above.