Abstract: The main purpose of the paper is to study the morphology and variation of the air core in the gravity-fed three-product heavy medium cyclone, the unsteady Reynolds stress model was used to simulate the influence of the parameters and operating parameters of the first-stage cyclone on the air core. And then analyze the impact on energy consumption and separation. The results show that the internal air column of the cyclone is basically formed at 21.67s, and the second-stage cyclone is preceded by first-stage cyclone to form an air core. When the feed velocity is less than 4m / s, the air core cannot be formed in the cyclone. More than this critical velocity, a complete and stable air core can be form. The excessive feed velocity will reduce the volume percentage of air core in first-stage cyclone, and lightly increase volume percentage of air core in the second-stage cyclone. The diameter of the overflow pipe of first-stage cyclone is 380mm, the insertion depth of the overflow pipe is in the range of 410mm-610mm, the volume percentage of air core is lower, the air core is small and the stability is strong.