Abstract: Understanding gas–solid hydrodynamics, heat and mass transfer, and multiphase reactions is of great importance to the design of a large-scale fluid catalytic cracking (FCC) riser reactor. FCC catalyst particles in a large-scale (e.g., demo-scale or industrial-scale) riser reactor are generally simulated using Eulerian methods since Lagrangian approaches are often avoided because of their high computational cost. As a result, information about the flow at the particle scale is not considered. In this study, the catalyst behaviors in a large-scale FCC riser reactor are investigated with a particle-scale model that is based on the multiphase particle-in-cell (MP-PIC) scheme. Cracking reactions are taken into account through the incorporation of an eight-lump kinetic model. Numerical predictions are found to be in very close agreement with the available plant data. Detailed particle-scale information, including the particle trajectory, residence time (i.e., internal age), and coke content of the catalysts in the large-scale riser reactor, are fully quantified. Catalyst particles may be entrapped in the diameter-enlarged section, leading to high mean residence time (i.e., 1.67 s) and coke content (more than 3.0% of 3.20% catalyst particles therein), and thereby low catalytic activities (e.g., 0.77 at the height of 2.0 m). It is believed that these findings should help better understand the particle-scale physical and chemical phenomena in large-scale FCC multi-regime riser reactors and assist the design, scale-up, and optimization of similar industrial devices.