The compressed air flowing from the inlet of the air filter is introduced into the baffle. The deflector has evenly distributed helical blades like fan blades, forcing the high-speed flowing compressed air to rotate strongly along the tangential direction of the teeth. The liquid water oil and the large impurities mixed in the air are separated by strong centrifugal force, and are thrown onto the inner wall of the water cup and flow to the bottom of the water cup. The liquid water oil and the compressed air of the larger impurities are removed, and further filtered through the filter element to remove minute solid particles, and then the clean compressed air is discharged from the outlet. The umbrella-shaped flaps divide the water cup into upper and lower parts, and the lower part maintains the pressure static zone, which prevents the high-speed rotating airflow from sucking up the water and oil at the bottom of the cup. The water that collects at the bottom of the cup is drained from the drain valve. The air filter must be installed with the vertical cup down.