Abstract: The spacing of exhaust outlets is a key factor for the concentrated smoke extraction system, which impacts the smoke control effect. A 22 m-long tunnel model with 1∶20 scale was used to test the effects of a concentrated smoke extraction system with four bi-directional exhaust outlets opened symmetrically with a spacing of 3 m and 5 m, respectively. A contrastive analysis of the temperature beneath the tunnel roof, spread length and thickness of the smoke layer was then carried out with those two different spacings. Results show that while both of the spacings can meet the requirements of safe smoke extraction in tunnel fires, it is safer and more conservative to choose the 3 m spacing, which means the design of an exhaust outlet with 3 m spacing is better than that with 5 m spacing.