The invention relates to a preparation method of p-xylene. P-xylene is prepared from a macromolecular polymer, namely, waste PET through high-temperature hydrogenation degradation with RuSnPt as a catalyst. The preparation method comprises specific steps as follows: the RuSnPt catalyst and PET are added to a high-pressure kettle in the mass ratio being 1%-20%, hydrogen is introduced under the pressure of 3-7 MPa at the room temperature, and the mixture is subjected to a reaction at the temperature of 240-350 DEGC for 3-13 h. The high-pressure kettle is washed with a solvent after the reaction ends, PET and the catalyst which are not decomposed are removed, a product is separated and purified, and p-xylene is obtained. The reaction condition is easy to control, operation is simple and convenient, no solvents are used in the reaction, post-treatment is simple, and the yield and the purity of p-xylene are high. Besides, a new method for preparing p-xylene is developed, a new direction is provided for resource reutilization of waste PET, and the preparation method has a great significance in environment protection and sustainable development.