Abstract: During fires in long channels, due to the restriction of side walls, the fire-induced smoke will rapidly reach the fully developed phase, with the characteristic of an axial one-dimension horizontal movement. It is analyzed for both the relative shear movement at the interface between the upper hot smoke layer and lower cold air layer during the one-dimensional horizontal movement of smoke, and the resulting increase of mass flow rate of the smoke layer. With combining the variation law of smoke velocity, the expression of an air mass entrainment coefficient was established. A series of experiments were carried out in a bench-scale long channel, the parameters in the expression were measured, including the ambient air temperature, and the velocity, temperature and thickness of smoke layers at two reference positions. Based on these experimental results, the horizontal entrainment coefficient was obtained.