Abstract: The invention relates to a ring-slot type magnetic liner comprising guard plates, permanent magnetic bodies, binders and medium protecting layers, wherein, more than one dead slots are equidistantly arranged on the structure of said guard plates along the circumferential direction, a permanent magnetic body installed in each of said dead slots, and along the circumferential direction of the guard plate, one coil is set as N pole and the other coil is set as S pole. Each of the protecting layers is formed on the surface of the guard plate and is of annular shape in the circumferential direction. The invention allows the grinding operation to be carried out in the slots of the ring-slot shaped protecting layer which is formed from cracked grinding balls. The grinding balls contact the materials at an angle of about 120°, and the grinding efficiency is kept constant. Therefore the attritioning efficiency and yield may be increased.