The design and operation of the secondary air (SA) have a great influence on the axial distribution of particles, the structure of the redox atmosphere, and the gas-solid mixture diffusion in the circulating fluidized bed (CFB) boiler. In this paper, the effects of the SA on the gas-solid flow in the CFB boiler, such as the axial particle distribution and the circulating rate were studied systematically through experimental analysis. All experiments were carried out under the ambient temperature and atmospheric pressure, and quartz sand was selected as the bed material in the 4.5 m high CFB test rig made of Plexiglas. The results showed that the SA whose capability to carry particle is affected by the particle concentration at the SA injection port, has a weaker effect on the entrainment of particles than the primary air (PA). The particle concentration in the dense phase zone mainly depends on the PA rate. The increasing of the secondary air ratio (SAR) may lead to the accumulation of particles in the dense phase zone. In addition, one transitional state was found in the transition zone. The variation of particle concentration in the transition zone with the increasing of the SAR is determined by the magnitude of the particle concentration at the SA injection port. The circulating rate decreased with the increasing of SAR or the SA injection height. The effect of the SA injection on the circulating rate in a circulating fluidized bed can be predicted by a modified model with the dimensionless parameters. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.