Abstract Critical back pressure plays a crucial role in the performance of ejector systems; however, few systematical investigations have been conducted to study an ejector operating near the critical point. In this paper, the effects of critical back pressure with variations of primary pressure on ejector performance were investigated in an ejector refrigeration system. Furthermore, an optimum primary pressure model was proposed, based on experimental results and validated by seven groups of experimental data reported in the literature. An optimum entrainment ratio model, verified by the same experiments, was proposed to predict the entrainment ratio of the ejector that operates at critical back pressure. The minimum coefficients of the determination of the two models are 0.9871 and 0.9625, respectively.