A dispersion reactor model of both riser and downer reactors for the fluid catalytic cracking is proposed. The model combines the four-lump cracking kinetics, the hydrodynamic model for both the riser and downer, and the dispersion mixing model to predict the fluid catalytic cracking reaction performance. The results show that the axial gas backmixing has a large influence on the gasoline yields of the reactor. When the axial Peclet number changes from 0.1 to 1000, the yield of gasoline will increase approximately 11% under the same conversion. Considering the large reduction of axial gas backmixing in the downer compared with that in the riser, the gasoline yield can be increased 5.5% for the downer reactor, which is consistent with the experimental result. It is found that the overcracking rate will greatly influence the reaction performance in the riser. The plug-flow assumption for the riser reactor is oversimplified in the catalytic cracking process.