To analyze the gas flow in an axis-in cyclone separator, a two-stage turbulence model based on the RNG κ-ε model combined with the Reynolds stress model was developed. The RNG κ-ε model was first used for the gas-phase of five cases of the cyclone. When the gas flow field was stable, the particle phase was added to the gas-phase and the Reynolds stress model was used for the gas-phase. A stochastic trajectory model was employed for the particle phase on the basis of the gas flow field, and the resistance and efficiency of the cyclone were calculated. Finally, a typical case was measured. The results agree reasonably well with the numerical results. Therefore, the simulation method based on the two-stage turbulence model is reliable for the study of the gas flow in axis-in cyclone separators.