The invention discloses a method for coating a reflection cup of a car lamp. The method is characterized in that the method comprises the following steps: (1) activation and cleaning treatment: carrying out activation and cleaning treatment on the surface of the reflection cup, wherein the treatment time lasts for 80-120 S, the vacuum degree in a vacuum chamber reaches 45-60 Pa, and the voltage of a coating electric cabinet output to the vacuum chamber through voltage transformation is 3,000V; (2) evaporation coating of an aluminum film: the aluminizing thickness is not more than 300nm; and (3) coating of a protective film: the voltage of a coating electric cabinet output to the vacuum chamber through voltage transformation is 3,000V, and the thickness of the coated protective film is 2.5-3.0 micrometers. As the method is used, the protective film has acid resistance and alkali resistance and does not fall off after being dipped in hydrochloric acid of concentration 1% or sodium hydroxide of concentration 1% and in water of 50 DEG C for 24h; and as a vacuum coating technology is used, the problem of nonuniform film thickness caused by manual spraying is effectively solved and the quality of the car lamp is greatly improved.