Solids back-mixing significantly influences the performance of circulating fluidized bed (CFB) risers. A better understanding of the solids back-mixing behavior is greatly essential for the design and operation of CFB risers. Computational particle fluid dynamics (CPFD) modeling based on the multiphase particle-in-cell (MP-PIC) method was applied to investigate the solids back-mixing behavior and its influencing factors in CFB risers operating in dilute phase transport (DPT) and fast fluidization (FF) regimes. The present work observed extremely long residence time and wide residence time distribution (RTD) curves for particles in the FF regime, indicating an extensive back-mixing of particles in the FF regime. The overall and local back-mixing behaviors of solids were further analyzed. The particles in the DPT regime have a little back-mixing in the lower part of the riser, while the particles in the FF regime experience a large-scale back-mixing throughout the riser. In addition, the factors influencing the back-mixing behavior of solids in different flow regimes were investigated. The results demonstrate that the solids back-mixing in the DPT regime is caused by the downward flow of particles near the wall region, while the severe back-mixing in the FF regime is due to the downflow particle's and mesoscale structures (mainly particle clusters). The dynamic formation and dissolution of mesoscale structures could be the dominating factor, leading to the intense back-mixing for particles in the FF regime.