Abstract: Fluid catalytic cracking (FCC) process is one of the most important heavy oil upgrading techniques in the petroleum refining industry,and riser reactor is the crucial part in FCC unit. An axial Particle Backmixing Model is established.By using two experimentally regressed parameters and the axial Particle Backmixing Model,the flow parameters of the particle phase such as mean density,mean axial velocity can be predicted well. The method for calculating radial distribution of nozzle jet in different riser cross-section is obtained,which considers both the Two Phases Diffusion Model and experimented results.It is also found that the ratio of pre-lift gas velocity and nozzle jet velocity is the decisive factor in this distribution while other operation parameters are secondary relatively.