The invention discloses PET (polyethylene glycol terephthalate) fiber/polyacrylonitrile fiber composite sound absorbing cotton, which is prepared from the following ingredients in parts by weight: 70 to 80 parts of PET fibers, 40 to 50 parts of polyacrylonitrile fibers, 10 to 15 parts of emulsifying agents OP-20, 5 to 10 parts of peregal O-15, 80 to 100 parts of PP (propene polymer), 30 to 40 parts of SEBS (styrene ethylene butylene styrene), 15 to 20 parts of calcined clay, 10 to 15 parts of modified kieselguhr, 2 to 3 parts of stearic acid, 5 to 8 parts of diisooctyl phthalate, 4 to 6 parts of tricresyl phosphate, 2 to 3 parts of polyethylene wax, 1 to 2 parts of vinyl (beta-methoxyethoxy) trisilane, 2 to 3 parts of calcium stearate, 1 to 2 parts of dimethyl dimercapto isooctyl acetate tin, 0.5 to 1.5 parts of anti-aging agents ODA (octyl decyl adipate) and 1 to 2 parts of antioxidants 1035. The sound absorbing cotton prepared by the invention has the advantages that the appearance evenness is good, the thickness is uniform, the hand feeling is soft, the product bonding degree is good, the tensile performance is high, the sound absorbing performance is excellent, the flame retardant effect is good, and the like.