In order to explore the influence of visual lag occurs to drivers in tunnel portal section on driving safety, a laboratory test model is established based on a great deal of tunnel investigation, and the visual lag test is carried out on driver driving in tunnel. And then, the difference of data increment is analyzed by T tests, and the visual lag conditions and causes are analyzed and proposed. The results show that: (1) When driving into the tunnel, the cognition lag occurs when brightness variation rate reaches 350 cd/m2 · s, and the regulation lag occurs when brightness variation rate reaches about 400 cd/m2 · s; while, when driving out of the tunnel, the cognition lag occurs when brightness variation rate reaches 400 cd/m2 · s, and the regulation lag occurs when brightness variation rate reaches about 500 cd/m2 · s. (2) The cognition lag has a certain influence on driving safety, and the regulation lag has more adverse effects on driving safety. (3) Some methods， i.e. extending driver′s visual adaptation time by using dimming facilities, refining lighting grading and stepless dimming, are suggested to slow down the brightness variation rate, avoid the visual lag phenomenon and improve the driving safety.