This paper presents an experimental study on the hydrodynamics and mixing behaviors in a riser-downer-coupling reactor proposed for the fluid catalytic cracking (FCC) process. Special attention is placed on two parts: the annular riser and the riser-downer junction. In comparison to the conventional riser, the annular riser has a more uniform radial flow structure, which can be observed from the radial distribution of particle velocity and solid concentration; however, the axial particle mixing seems to be similar. The carefully designed junction can serve as an inlet structure of the downer part to realize the uniform distribution of solids, while it does not increase the back-mixing of particles of the whole coupling reactor. The particle dispersion mechanism has also been investigated, and the differences in the microstructures can explain the characteristic particle mixing behaviors observed in the coupling reactor.