The invention discloses a light adjusting system and method for an extending section outside a highway tunnel portal. A sunshade comprises a supporting frame and an outer cover plate fixed to the supporting frame. The sunshade is divided into a plurality of sections in the road direction. Shading piece supports are arranged in the sections correspondingly and fixed to the inner side face of the outer cover plate; a plurality of shading pieces are movably connected in the shading piece supports in sequence in the road direction; driving devices used for driving the shading pieces to rotate are fixed to the shading piece supports; a first illumination sensor used for detecting the illumination intensity outside the outer cover plate is arranged on the outer wall of the outer cover plate; a second illumination sensor used for detecting the illumination intensity of the tunnel portal is arranged in the tunnel portal; and third illumination sensors used for detecting the illumination intensity in the sections are arranged in the sections correspondingly. According to the system and method, transition between the high illumination intensity and the low illumination intensity in the extending section of a tunnel can be achieved in a self-adaptive mode, light adjusting flexibility is high, building and maintenance costs are low, and construction difficulty is low.