Semi-countercurrent drum magnetic separator (SDMS) is a key low-intensity magnetic separator, it boasts advantages such as simple operation, low maintenance costs, and consistent separation efficiency, etc., but few research papers have reported the mechanism of its separation process. As the feeding size gets smaller, highly efficient recovery of fine minerals becomes a new issue faced by the SDMS. In this study, the COMSOL Multiphysics software was used to establish a simulation model for the SDMS process, to reveal its flow field distribution, magnetic field distribution, and particle trapping characteristics. The simulation results show that in the SDMS process, the pulp flow rate is low before entering into the separation zone of the separator, but it turns faster when passing through the separation zone, and a fast-flow layer existing in the vicinity of the drum helps to disperse the pulp and improve selectivity. The N-S poles arrangement of the permanent magnet blocks in the magnet system makes the magnitude and direction of magnetic induction on the drum surface change rapidly, which not only intensifies the magnetic force acting onto magnetic particles, but also benefits the rolling of captured particles on the drum surface, and further improves the selectivity. Investigation on particle capture law under different operation conditions found that recovering -20 μm magnetite is very difficult, but increasing the magnetic induction, reducing the feeding flow rate, and shrinking the separation gap helps to intensify its recovery. Since a low feeding rate means low throughout, a combined strategy of a high magnetic induction, a small separation gap, and a suitable feeding rate is recommended to enhance the recovery of fine magnetite. After using this strategy, the recovery of a cyanide tailing was dramatically enhanced from less than 70% to above 90%.