An aerobic moving bed biofilm reactor (MBBR) was applied to treat landfill leachate generated from a domestic waste incineration plant. Pollutant removal efficiency of this reactor under stable operating condition was studied. The biomass, bacteria species, and microbial metabolism in this reactor were investigated. These results showed that the average removal efficiency of chemical oxygen demand (COD) and ammonia nitrogen in the aerobic MBBR achieved 64% and 97% in the optimum conditions, respectively. The three-dimensional fluorescence spectrum revealed that the content of soluble microbial byproducts from extracellular polymeric substances extraction in suspended sludge was much higher than that on biofilm, and the types of pollutants were various in different regions of the reactor. It also indicated that the MBBR system had a stable, rich and regular microorganism community, including large amounts of nitrifying bacteria and denitrifying bacteria. Scanning electron microscopy suggested that biofilm attached to the packing provided a good anoxic–aerobic micro environment system to achieve a high metabolic activity, which favored COD and ammonia removal.