Abstract: The mathematical model for the gas-solid adsorption mass transfer in a cocurrent downflow circulating fluidized bed(CDCFB) reactor is proposed in this paper. Experiments were conducted for measuring the axial tracer concentration distribution by using the steady-state tracer method. The model parameters, mass transfer coefficient and axial diffusion coefficient were obtained by non-linear retrive method (Marquardt method).The empirical correlations of experimental data for model parameters were obtained, as given by the following equationsPe2 =The model can predict the characteristics of gas ?solid mass transfer and fit the experimental results very well. So the model proposed in this paper could be used to describe the mass transfer in CDCFB.