In order to improve the driver's safety in expressway tunnel groups,the eye-movement equipment is a- dopted to conduct a pupil variation experiment,and the relationship between the driver's pupil area and the ambient illumination as well as the longitudinal depth of the tunnel is analyzed.Then,a“six small”division method of tun- nel groups is proposed,and a curve model describing the relationship between the maximum difference of the driver's pupil areas and the connection length of the tunnel is constructed.The results indicate that there exists an exponential relationship between the driver's pupil area and the ambient illumination,and the visual light adaptation level and connection length of the upstream tunnel exit are the main factors influencing the visual adaptation of both the downstream tunnel and the whole tunnel group sections.Finally,the recommended values to improve the am- bient illumination of the tunnel group are put forward.It is suggested that,when the tunnel spacing is less than 50 m,the shading shed in the tunnel connection section or the illumination gradient exit of the upstream tunnel should be set,while within the range of 50 ～100m,the illumination gradient exit of the downstream tunnel should be set.