Abstract: Aiming at the production technical problems such as ore body scale reduction，low ore re⁃ covery rate and high waste rock mixture rate existing in the transition stage (-430~ -570 m) from the Ⅲ# main ore body to the Ⅵ# ore body of Chengchao Iron Mine，the step distance optimization research of the sublevel caving method without sill pillar of the ore body in the transition section of Chengchao Iron mine was carried out. A two-dimensional ore discharge model was constructed by using the discrete element nu⁃ merical simulation platform. Five ore-breaking step distance schemes ranging from 3.2 m to 4.8 m were set up and a comparative analysis was conducted to systematically study the influence law of the stope structure parameters on the ore-discharging indicators. Based on the Bergmark-Roos drawing theory，the theoretical value range of the ore collapse step distance was initially determined，and then the PFC2D numerical simula⁃ tion software was used for optimization and verification. The research results show that the step distance val⁃ ue determined based on the Bergmark-Roos drawing theory is 4~4.4 m. The numerical simulation results show that as the step distance of ore collapse increases，the recovery of ore first increases and then decreas⁃ es，and the mixture of waste rock first decreases and then increases. The 4.4 m mine collapse step distance can achieve the collaborative optimization of mining parameters. The ore recovery rate can reach 85.95%， and the waste rock mixing rate is 15.53%. The research results provide guidance for the optimization of the stope structure parameters in the transition section of Chengchao Iron Mine.

Key words: Bergmark-Roosdrawing theory, poverty return indicator, PFC2D numerical simulation, cav? ing step distance