The utility model provides a rock wool cotton waste reducing mechanism can carry out the grinding to the rock wool cotton waste, guarantees that the rock wool cotton waste can be by abundant recycle or as the spraying raw materials. A rock wool cotton waste reducing mechanism, includes that power transmission mechanism, rubbing crusher construct and support guiding mechanism, rubbing crusher constructs slip grinding roller and fixed grinding roller including front and back parallel arrangement, and power device connects fixed grinding roller, support guiding mechanism including fixed rolling bearing unit, slip rolling bearing unit and guide rail, fixed rolling bearing unit's bearing is fixed at fixed grinding roller both ends, and the connection can be dismantled on the guide rail to fixed rolling bearing unit's bearing frame, and slip rolling bearing unit's bearing is fixed at slip grinding roller both ends, and slip rolling bearing unit's bearing frame can be followed the guide rail and slided, the perpendicular both sides that set up at the rubbing crusher structure of guide rail.