Mixing performances of municipal solid waste (MSW) in differential density fluidized beds (FBs) have been investigated simulating five kinds of materials (plastic, wood, candle, coal and coal-stone). The results show that the differential density FB technology can realize good mixing performances for different MSW components. The density of MSW components has a conspicuous effect on the mixing characteristics of FB. Also, the size of the components has effects. The mixing index is adopted via a regression analysis to the experimental data, and a mixing expression formula is obtained, that will be beneficial for MSW incinerator design and operation.