Gas-liquid separator plays an important role in the reactor fission gas removal system. Utilizing the differential pressure formed by flow around the perforated tube, the separator achieves gas–liquid separation, meanwhile having advantages such as simple structure. The evolution characteristics of gas–liquid interface and the variation of parameters such as separation efficiency and pressure drop under plug flow and slug flow are investigated. The void fraction in wake region is only related to the inlet volumetric fraction. The variation of air separation efficiency with inlet volumetric fraction is influenced by the inlet flow pattern, and the separation purity increases with the inlet volumetric fraction. The prediction methods of air separation efficiency, pressure drop and other parameters based on Weber number and liquid phase Reynolds number are obtained in accordance with dimensional analysis and minimum kinetic energy principle with the prediction accuracy within ±20 %, which provides guidance for practical application and optimization.