This mechanism employs an inlet support, which is located at the front side of the plastic grain-collector. A second plastic grain-collector is installed on the right side of the plastic grain-collector and the second plastic grain-collector is also a circular-tank-shaped container. The rotary cutter-holding base is located at the bottom of the collector, and there are three cutters held onto the rotary cutter-holding base. Three cutters for pulverization installed on the top of the cutter-holding base pulverize the plastic grains for the first time; the plastic grains are pulverized for the second time in the main plastic grain-collector. The cutting mechanism pulverizes the plastic grains with the help of the anti-gliding cutters at the inner wall of the plastic grain-collector, and by which it increases the performance of the pulverization. At last, by pushing the force induced from the rotation of the cutter-holding base, the plastic grains will move smoothly into the inlet. The main feature is that the raw materials processed initially in the second plastic grain-collector are pulverized thoroughly in the main plastic grain-collector. Since the performance of pulverization is improved, the better quality of the end product can be obtained.