In this paper, the pan-milling process and the pulverizing mechanism and efficiency of pan-mill type equipment, which was designed and manufactured in our lab for pulverization, mixing, and stress reactions of polymers, have been studied. The effects of load and rotation speed of the equipment on the pulverizing of polystyrene was investigated by means of quantitative operation, particle size, and distribution measurement, as well as theoretical analysis using the Rosin-Rammler Function and the logarithmic normal distribution function. The results demonstrate that lower rotation and higher load are favorable to higher pulverizing efficiency and to the mode of surface pulverizing. As a result of the quite strong forces during pan-milling mechanochemical degradation takes place, which is a first-order reaction.