The particle mixing behavior has been investigated experimentally in a spout-fluid bed. The bed material used is polypropylene particles, and the tracer employed is chosen from silica gel, mung beans, and glass beads. During all experiments, the completely segregated arrangement of particles is adopted as the initial packing condition. The gas velocity varies to cover internal jet, minimum spouting, and fully developed spouting conditions, in order to establish a full mixing map about the effect of particle density. The mixing behavior is analyzed in terms of flow patterns, concentration profile, and mixing index. The results show that the degree and rate of mixing are significantly affected by the particle density, and heavier particles achieve a higher mixing rate but a poorer mixing quality. The mixing map reveals that to reach the same mixing index, the spouting gas velocity needs to increase with the increase of particle density. Besides, provided that the jet gas velocity does not exceed the minimum spouting gas velocity, a segregation phenomenon can always be observed by simply adjusting the ratio of the fluidizing to spouting gas velocity.