As a new form of urban traffic, extra-long undersea tunnels have emerged as an important means of transportation to promote the development of coastal cities in recent years. Owing to the black hole effect and the special alignment at the entrance of these tunnels, the occurrence of a traffic accident can often lead to casualties and traffic paralysis. This study analyzed the main characteristics of the tunnel entrance length division methods proposed by the International Commission on Illumination (CIE), the United States, and China, and discussed the main differences between the lighting environments of extra-long undersea tunnels and ordinary highway tunnels. A real vehicle test was carried out in an extra-long undersea tunnel, and primary data, such as the driver’s pupil area and tunnel illuminance, under continuous time series were collected. The drivers’ pupil variation characteristics while driving into the tunnel during the daytime were studied, and a mathematical model of the change in the pupil area with time was constructed. In addition, a method for calculating the drivers’ dark adaptation time at the tunnel entrance was established, with the pupil area growth rate and volatility less than 15% as the indices. Based on the driver’s visual adaptation characteristics and stopping sight distance, a method for the entrance zone length of the extra-long undersea tunnel was proposed. The lengths were enhanced by 41.176, 34.513, and 20.635%, respectively, compared to those obtained using the CIE/USA/CHN standards. In view of the different calculation methods and results of the existing standards, our method overcomes the limitations of the existing methods that rely excessively on the physical and lighting characteristics of tunnels. Moreover, it considers the potential safety hazards where the main road and the ramp merge near the entrance of the tunnel, and provides a new idea for the safe length of the entrance zone from the perspective of the driver’s visual physiological response.