Abstract: With a mixture of oxygen and nitrogen as the subject investigated, the direct simulation Monte-Carlo method was applied to simulate the flow of paramagnetic and diamagnetic mixed gas in high gradient magnetic fields. The simulated results were compared with the existing experimental ones, and the reliability of the program was demonstrated. It was shown that the increment of oxygen concentration increased from 0.12% to 1.89% when the product of magnetic field and gradient varied from 100T²·m^-1 to 1 000 T²·m^-1. The increment of oxygen concentration reduced from 0.4 % to 0.1% when the temperature varied from 273 K to 333 K. The increment rose from 0.12 % to 0.59 % when the initial oxygen concentration varied from 20 96 to 30 %. The change in pressure did not almost influence the oxygen concentration.