NOVELTY - Modifying polyethylene (PE) film and polytetrafluoroethylene (PTFE) film composite film comprises infiltrating PE film in a modifying solution comprising paraffin oil, petroleum ether, gasoline and/or aeronautical kerosene, compositing infiltrated PE film with PTFE film in a multi-roller composite device, drying, heating, drying, stretching in a multi-roller tension device, extracting the stretched composite film in an extraction device, where the extraction agent comprises e.g. ethanol, acetone and poly trichloroethylene, blast drying and cutting the finished product.    USE - The modified PE film and PTFE film composite film is useful in lithium ion batteries (claimed).    DETAILED DESCRIPTION - Modifying polyethylene (PE) film and polytetrafluoroethylene (PTFE) film composite film comprises (a) infiltrating PE film in a modified solution at 35-85 degrees C for 10-60 minutes until the PE film is completely wet, where the total modified solution absorbed content is 10-55% and the modified solution comprises 15-20 pts. wt. paraffin oil, 25-32 pts. wt. petroleum ether, 25-40 pts. wt. gasoline and/or 35-50 pts. wt. aeronautical kerosene, (b) compositing the infiltrated PE film with PTFE film in a multi-roller composite device at 65-125 degrees C at a pressure of 0.05-20 MPa at a composite roller rotating speed of 0.1-10 m/minute, (c) taking out the composite film, drying at 35-65 degrees C in a clean sealed constant temperature drying tank, heating to 65-125 degrees C at a rate of 0.5 degrees C/minute, drying at a constant temperature for 10-3000 minutes, (d) stretching the dried composite film in a multi-roller tension device at 35-95 degrees C with a stretching ratio of 1:1.2-1:5, (e) placing the stretched composite film in an extraction device at 25-45 degrees C, extracting for 5-90 minutes, where the extraction agent comprises 5-15 pts. wt. ethanol, 18-32 pts. wt. acetone, 21-40 pts. wt. poly trichloroethylene, 25-49 pts. wt. N,N-dimethylformamide and 30-50 pts. wt. N,N-dimethylacetamide, (f) placing in a constant temperature blast drying device, blast drying at 45-100 degrees C at an air rate of 100-1500 revolutions per minute for 25-125 minutes, cutting the finished product.