Abstract: Brightness L20(S) outside a tunnel, as the basic parameter of lighting at entrance section of a highway tunnel, affects the lighting design of the transition section, and its value is subject to the combined action of various factors. Many factors affect the measurement of brightness outside a tunnel. This paper studies the influence of each factor on the brightness L20(S) value outside the tunnel based on three factors, i.e. the sky exposure rate outside the tunnel, the form of the tunnel portal and the color of the pavement surface outside the tunnel. Through investigating highway tunnels within the main city of Chongqing, corresponding tunnels which meet the requirements are selected as the research objects. In combination with theoretical analysis and field test verification, the influence law of each index parameter on the brightness L20(S) outside the tunnel is obtained and the indices of brightness outside a highway tunnel are put forward based on field brightness test method. The study results show that: (1) the recommended value of the brightness L20(S) outside a tunnel in autumn and winter should be at least one time and above the actual measured brightness value; (2) the patterns of the values of pavement brightness are consistent with that of the RGB values; and (3) the brightness outside a tunnel with end-wall type portal is smaller than that with bamboo-truncated type portal.