Abstract: The pre-concentration of comminuted products by high-pressure grinding roller for lean hematite was studied, using the cylindrical ferromagnetic medium. The effects of various factors including rod diameter, rod gap and background magnetic field intensity (BMFI), on indexes of pre-concentration were emphatically investigated and the force analysis of single mineral particles with different diameters in the separation zone was done. The results showed that as rod gap decreased, rod diameter and BMFI increased, the tailings grade and the tailings yield are reduced, but the Fe recovery is increased. Under the same BMFI, the magnetic capturing force increases on the coarse particles and decreases on the fine particles with the rod diameter increasing. For the actual locked particles, considering the gravity and viscous resistance of particles, the increase of rod diameter is beneficial for the capture of coarse particles but has no influence on the capture of fine particles, while the decrease of rod diameter is good for the capture of fine particles but bad for the capture of coarse particles of lean intergrowth.