The objective of this paper is to study the gas-liquid separation performance and its improvement of a new combined T-junctions separator by experiments. This separator is constructed of large-diameter T-junctions (50 mm and 60 mm), symmetrically arranged in annulus. The test conditions include complete separation and incomplete separation. The gas-liquid flow characteristics of the combined T-junctions were visually observed and analysed during the separation process. The separation characteristic was quantified (F) and the effects of liquid level, inlet superficial velocities (JG1 and JL1) and outlet valve opening degree on the separation efficiency (F) were confirmed. The results show the connections between the parameters and separation efficiency. Based on the above, a self-regulating level controller is designed, which can significantly extend the range of inlet gas velocity (JG1) to 4.5 times and inlet liquid velocity (JL1) to two times for complete separation, but the level controller fails when the inlet gas-liquid velocities are too high (JG1 > 10.27 m/s, JL1 > 0.52 m/s). [Received: December 14, 2021; Accepted: August 7, 2022]