Computational particle fluid dynamics (CPFD) simulation was carried out to analyze flow pattern in order to understand the mechanism of deposit formation on the cyclone duct during the residue fluidized catalytic cracking (RFCC) process. The CPFD simulation was based on the multiphase particle-in-cell (MP-PIC) method, in which the particle phase was solved by the stochastic Largrangian model and the fluid phase was analyzed by the Eulerian method. In the first stage, a low-speed zone at a duct angle of 90 degrees (an inlet position of 0 degrees) was discovered in the basic design of the cyclone, and this zone was predicted to be the initial region of deposit. Case studies were carried out for several stages of deposit growth is expected to occur when the deposit reaches 90 mm in thickness. When the deposit reaches 150 mm, the carryover rate abruptly doubles.