Abstract: In the event of core disassembly accident (CDA) in a sodium-cooled fast reactor (SFR), the molten material from the core migrates and interacts with the low-temperature coolant in the lower plenum, resulting in the formation of core debris that eventually settles to forms a debris bed. Improper placement and inadequate cooling of the debris bed can result in the failure of the pressure vessel. To efficiently disperse the core debris and minimize contact between the lower head of the pressure vessel and the molten material, a core catcher was specifically designed as a passive preventive and mitigating device. The structure of the core catcher directly impacts the shape and distribution of the debris bed, which subsequently affects its re-criticality and long-term decay heat removal capability. This study aims to perform a numerical investigation to optimize the structural design of the core catcher, with a specific focus on the mechanisms and laws of the chimney structure of the core catcher on the formation and distribution of the debris bed. The improved discrete element method (DEM), which is based on dimensionless stiffness and dimensionless damping coefficients, was employed to numerically simulate the formation process of the debris bed on the core catcher. This method incorporates source terms into the momentum equation of debris particles to account for the influence of coolant on their motion. Validation experiments were conducted to verify the capability of the numerical simulation algorithm used in optimizing the design of the core catcher. In this paper, the effects of three factors on particle motion and debris bed formation mechanisms were investigated by varying the vertical projection side length of the chimney cover, inclination angle of the cover, and spacing between chimneys. The research findings suggest that the selection of appropriate values for these parameters has a substantial impact on the shape and distribution of the debris bed. Therefore, it is recommended to establish reasonable chimney design parameters to achieve optimal performance in practical applications. Furthermore, this study reveals that during the formation of the debris bed, the debris particles demonstrate a secondary scattering effect, which plays a vital role in enhancing the uniformity of the debris bed. This study will provide valuable engineering references for optimizing the design of the core catcher.