A closed ultrafine grading system developed for the classification requirements of special materials such as flammable, explosive, and easily oxidizable materials, utilizing inert gas circulation protection. By replacing the oxygen in the system with inert gas, when the oxygen content is lower than the required calibration value, the material is transported to the classifier. The high-speed rotating classifier rotor generates a swirling centrifugal force, which sorts the coarse particles to the inner wall of the chamber, slows down and falls down along the cylinder wall to the discharge port for discharge. At the same time, the fine powder passes through the gap between the classification wheel blades and enters the cyclone collector or directly enters the dust collector, thus achieving the purpose of separating coarse and fine materials.