A method for preparing a polymer nanocomposite is provided with steps of (1) dissolving polyvinyl chloride; (2) dissolving polyethylene; (3) dissolving a polyvinyl chloride-polyethylene block copolymer; (4) adding a surfactant into a mixed solution obtained from the above-mentioned steps; (5) adding a light permeable material into the mixed solution; (6) adding an enforcement material into the mixed solution; (7) performing an ultrasonic mixing to the mixed solution; and (8) performing a rotary evaporation to the mixed solution.