In this paper, the mixture composition separation performance of a compound T-junction for vapor–liquid two-phase zeotropic mixtures R134a/R245fa was experimentally investigated. The effect of inlet vapor quality and mass flow rate on the separation performance were investigated. The composition separation performance of compound T-junction and simple T-junction were compared. The separation models for simple T-junction and compound T-junction were also discussed. During the experiments, the composition ratio of the zeotropic mixture was 0.429/0.571 (R134a/R245fa). In order to represent the degree of actual separation to the ideal separation, the composition separation efficiency was defined as the ratio of the actual degree of composition separation to the ideal degree of composition separation. The inlet vapor quality varied from 0.09 to 0.19, and the composition separation efficiency increased from 38.34% to 86.86%. When the mass flow rate increased from 10 g/s to 13 g/s, the composition separation efficiency could be increased by up to 40%. Compared with simple T-Junction, composition separation efficiency of compound T-Junction was improved by about 10%. Furthermore, the separation model of the compound T-junction was developed based on the experimental results. And the coefficient of determination is 0.9662.