The invention discloses PET (polyethylene glycol terephthalate) fiber / polyimide 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 polyimide fibers, 10 to 15 parts of emulsifying agents OP-15, 5 to 10 parts of peregal O-10, 80 to 100 parts of PP (propene polymer), 30 to 40 parts of SBS (styrene butadiene styrene block polymer), 15 to 20 parts of nanometer dolomite powder, 10 to 15 parts of modified kieselguhr, 2 to 3 parts of stearic acid, 5 to 8 parts of dibutyl phthalate, 4 to 6 parts of triethyl phosphate, 2 to 3 parts of ethylene bis oleamide, 1 to 2 parts of tetraisopropyl di(dioctylphosphate) titanate, 2 to 3 parts of calcium stearate, 1 to 2 parts of lead sulfate tribasic, 0.5 to 1.5 parts of anti-aging agents MB and 1 to 2 parts of antioxidants 1010. 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.