The high-temperature toxic gas released by long railway tunnel fires not only causes great harm to persons, but also damages the structure of the tunnel which will reduce the overall stability of tunnel. In order to diminish the damage to tunnel structure produced by a tunnel fire, on the basis of the first extra-long underwater railway tunnel in China, some reduced-scale tests were carried out to study the distribution of smoke temperature along the tunnel ceiling, the smoke velocity and the backlayering distance with the fire size of 63KW. The longitudinal ventilation velocity and the tunnel gradient varied in these tests. The smoke temperature below the tunnel ceiling in different times and under different longitudinal ventilation velocity, the smoke velocity under the ceiling, and the backlayering distance in the presence of different ventilation velocity are acquired from the tests. The conclusions have the guiding meaning to the disaster prevention design and construction of structure fire safety in tunnel fires, and all the experimental data presented in this paper are applicable for the verification of numerical models.