Due to tremendous differences of brightness, the tunnel portal is always the place where high incidences of traffic accidents occur. If some light reduction measures were taken in the section close to the tunnel portals, it would form an optical transition zone which could optimize driving environment and ensure traffic safety. Through analyzing related researches of light reduction measures applied in highway tunnel portals, this paper introduces and summarizes the types, merits and demerits, and application scope of light reduction measures. In addition, combining with the domestic application status of light reduction measures utilized in highway tunnel portals, the paper brief ly generalizes some considerable factors of light reduction measures in practice.