The continuous dimming of the illumination at the entrance section of the highway tunnel is critical to realize illumination on demand under the premise of safety. Regarding the problems of brightness jump and brightness insufficiency frequently appearing during the dimming of highway tunnel enhanced illumination, this paper uses the adaptation luminance change profile recorded by the LED intelligent fine dimming illumination system of Caihong expressway extralong tunnel in Hangzhou, to fit into a theoretical dimming profile, and make graphic comparable research with the real dimming profile. It reveals the reasons causing brightness frequently jump and brightness lower than the standard requirement, and proposes methods to improve dimming safety, and draw the following conclusions: 1) With higher level of dimming brightness and shorter interval, the amplitude of brightness jump at the tunnel entrance section will be smaller and the traffic is more safe; 2) The fineness of illumination dimming system and interval of dimming operation are important indexes of system safety.