Catalytic cracking reactors serve as crucial facilities for enhancing heavy oil utilization through light olefin production, where gas-solid hydrodynamics significantly influence reaction efficiency. This study investigated complex inter-phase and intra-phase interactions and flow behaviors in a diameter-transformed fast fluidized bed catalytic cracking reactor using a two-fluid model approach. The numerical model was validated against experimental measurements, confirming its accuracy in capturing gas-solid flow characteristics. Systematic analyses were conducted to examine the effects of nozzle gas velocity and structural parameters on particle dynamics. Results demonstrate that the increased gas velocity effectively mitigates particle accumulation in the sudden expansion zone, promoting more uniform solid distribution within the reactor. However, gas velocity variations showed negligible impact on particle concentration profiles in upper regions. Structural optimization revealed that implementing a conical transition with specific inclination angles between the upper and lower sections of the gradual expansion zone enhanced particle distribution uniformity and gas-solid mixing efficiency. These improvements were particularly conducive to optimizing catalytic cracking processes. These findings provided theoretical guidance for reactor design aiming at enhancing fluidization quality and process performance.