Abstract: Gas-solid fluidized beds, due to their enhanced gas-solid contact and improved mass and heat transfer efficiency, have been widely adopted in various natural uranium conversion processes. However, research on the fluidization behavior of high-density particles remains limited, and the underlying fluidization mechanisms have yet to be systematically elucidated. This knowledge gap impedes the precise design, optimization, and scale-up of such systems. For example, in processes such as reduction and hydrofluorination, the particles involved exhibit significantly higher densities (referred to as high-density particles) compared to conventional gas-solid fluidized beds. This results in suboptimal fluidization dynamics and reaction performance. In these countercurrent gas-solid contact systems, the irregular and unpredictable particle motion complicates the prediction of fluidization behavior and reaction kinetics. To ensure product quality, operators often resort to excessively high gas flow rates and prolonged particle residence times—an inefficient solution. Therefore, there is a pressing need to adapt research methodologies from other industries where gas-solid fluidization has been more extensively studied. In this paper cold-model fluidization experiments were conducted using actual materials to characterize fluidization dynamics. The observations reveal pronounced channeling phenomena, highlighting the necessity to optimize fluidization for high-density particle fluidization. Based on these, a computational fluid dynamics (CFD) model was developed and validated against experimental data. The results demonstrate reasonable accuracy in predicting key fluidization parameters, such as the minimum fluidization velocity U_mf. However, when the superficial gas velocity fell below U_mf, the pressure drop prediction relative error reaches about 24.5%. Given that the study focuses on macroscopic particle dynamics, this relative error is considered acceptable. While current fluidized bed designs have achieved partial success, further research is essential to develop simpler, more operable systems that meet industrial-scale productivity demands. This requires a fundamental understanding of the high-density particle fluidization mechanism. Key findings include that particles near the distributor plate loosen first, and gas gradually forms bubbles during fluidization. Due to the high particle density and bed height, gas compression effects occur, leading to bubble coalescence. When the superficial gas velocity is below U_mf, particles accumulate at the bottom, with only a few lifted by the gas. When the gas velocity exceeds U_mf but remains below U_ff, the bed height increases with gas velocity, particle distribution becomes uneven. When the gas velocity surpasses U_ff, the bed height stabilizes, and particles form an external circulation pattern, enhancing gas-solid contact. However, slugging and channeling occur, transforming the bed into a bubbling fluidized bed, which intensifies wall erosion and bed vibration. These results establish a theoretical foundation for systematically unraveling the fluidization behavior and mechanisms of high-density particle systems.