Abstract: Riser has the advantage of high gas-solids contact efficiency,high gas/solids flux and so on.But there is relatively significant gas and solids backmixing.On the ether hand,downer has the great advantage of uniform gas and solids residence time, but the entrance structure has great influence on its performance and the solid concentration is much lower than that in riser.A new type of Riser-Downer-Coupling Circulating Fluidized Bed (RDCCFB) is devised in this research, which is a close combination of riser and downer.This new type of CFB takes advantage of both riser and downer.Phosphor particles were used as tracers to study the solid mixing behavior in a cold-model RDCCFB.The results show that the overall Peclet Number is greater than that in a single riser.And the average residence time and the residence time distribution of the particles can be changed according to the requirement.These characteristics make this coupling reactor attractive in many areas.