In order to improve the uneven mixing of catalyst particles and feed in a feedstock injection zone of FCC Riser, the formation mechanism of the nonuniform mixing is studied through both experiment and 3-D CFD simulation over a large-scale cold-model riser. The effects of operating conditions on the mixing behavior are further investigated. Based on the formation mechanism, a novel scheme of feedstock injection is proposed and compared with the traditional scheme. The result indicates that the traditional scheme of feedstock injection leads to the uneven mixing. A desirable operating condition [Gs?80 kg/(m2?s), Ug?3.28 m/s, Ujet?62.5 m/s] can improve the distributions of solids and feed, reduce the axial range of catalyst-feed mixing by half, but the uneven mixing persist. The novel design especially the 30o downward scheme can be expected to realize a desired mixing.