The gas–liquid separator is a key component in the gas removal system in thorium molten salt reactor (TMSR). In this paper, an experimental study focusing on the gas core formation in the gas–liquid separator was carried out. We observed that formation of the air core depends primarily on the back pressure in the separator. Gas core formation was visualized for a range of back pressures, swirl numbers, and Reynolds numbers. Analysis of flow patterns indicated that gas core formation may be defined as four stages: “air core with suction,” “tadpole-shaped core,” “cloudy core,” and “rod core.” When rod core is achieved, gas bubbles will be separated completely and that particular back pressure is defined as critical back pressure. The critical back pressure depends on swirl number and Reynolds number. The trends how the critical back pressures vary with the Reynolds number and the swirl number were analyzed.