Abstract: Big differences of illumination intensities inside and outside of tunnel entrance is the main reason of traffic accident, and reasonable transition of illumination difference is available to guarantee traffic safety with dimming structure installed at tunnel entrance. Based on an investigation and study on the dimming structures for the tunnel entrance in China, the types, cross-sectional forms and application scopes of the dimming structures are summarized. The illumination test is designed to study the change rate of visual adaptability illumination when drivers arrive at the tunnel entrance, and a method to calculate dimming structure length is proposed considering the relevant tunnel lighting specifications. A dimming structure model is established by using luminous environment analysis software Ecotect and luminous environment simulation is carried out. The results show that the length of dimming structure installed at tunnel entrance should be 58 m at least when the drivers drive to tunnel at a speed of 100 km/h in condition of 60 000 lux ambient illumination.