The diffuser is the key part of a supersonic separator, which is a new device for natural gas separation. In this paper, three diffusers are designed for the supersonic separator. The fluid flow in the diffuser is numerical calculated, using the RNG k- turbulence model. The behavior of gas dynamic parameters is analyzed under conditions of shock waves and boundary layers. The numerical results show that the second throat diffuser is a good choice for the supersonic separator, both from the perspective of the pressure recovery and adjustment to the back-pressure changes.