In order to analyse the separation characteristics of the oil-gas separator under the extreme operating conditions (at -20 ℃, 0.2 MPa), the effects of the cylinder length, the diameter of outlet pipe, the distance between inlet and upper head and the setting of cyclone components on separation performance were numerically investigated. The results show that the longer cylinder weakens the swirl in the lower part of the separator. The separation performance is the best when the cylinder length is 950 mm. The pressure drop increases with a small diameter of the outlet or installing a spiral blade in the pipe. The increasing flow velocity results in more escaping liquid particles. The separation performance is the best when the diameter of outlet pipe is 60 mm. In addition, the oil-gas separator performs satisfactorily when the upper end of the guide blade is located below the inlet center-line and the distance equals the inlet diameter.