Longitudinal connecting flues connect the flues of the north and south line tunnels of the Qingdao 2nd subsea tunnel. This work is the first attempt in a highway tunnel. To examine the effect of connecting flues on the smoke exhaust of highway tunnels, various spacings between connecting flues are set to explore the distribution patterns of fire temperature field, smoke spread, and system resistance and determine the optimal spacing of connecting flues. Based on the evaluation indices, including smoke spread length and smoke exhaust efficiency, the resistance reduction rate is proposed for the construction of an effective evaluation model for the spacing between connecting flues. Furthermore, the configuration forms of various spacing settings between cross passages in China are investigated and the Fluent software is employed to simulate smoke flow characteristics at various connecting flue spacings. The results reveal the following: (1) The connecting flues effectively reduce the resistance of the smoke exhaust system. (2) With shortening of the connecting flue spacing, the smoke spreading length decreases from 260 to 180 m, the smoke exhaust efficiency increases from 93.39% to 97.80%, and the resistance reduction rate decreases from 15.42% to 2.20%. (3) The combined use of 750 and 1 500 m connecting flue spacings exhibit a better smoke exhaust effect than small spacing.