The invention relates to a preparation method of protecting film for car light, belonging to the technical field of vacuum coating, which mainly comprises the following steps: (1) pretreatment: to clean the surface of the car light base in ultrasonic cleaning machine with the mixture of cleanize liquid and purified water; (2) vapor deposition of aluminum film: to put the cleaned base into a vacuum chamber to vapor deposit the aluminum film; (3) preparation of soft protecting film: to converge by adopt the monomer material - hexamethyl-polysiloxane ion and generate plasma with the glow discharging in high pressure direct current field, of which the intensity is 2500 to 3500 V and the current intensity is 2 to 3 A; and the pressure of the vacuum chamber is 2 to 5 Pa when generating glow. The invention has the advantages that the monox protecting film formed by evaporation has good stability and a hard texture of the ceramic film so that the protecting film is stable in chemical propertyand can not destructure and become yellow in condition of high temperature and ultraviolet irradiation, which ensures the optical property of the car light; due to the adoption of the soft protectingfilm, namely the intermediate layer, the inner stress between the aluminum film and the monox protecting film is reduced effectively and the bonding force between the film layers is well ensured. Thereflectivity of the car light made by the invention is all above 90 percent, much better than the national standard of car light.