<p>Central ejectors have typically been used in high-altitude test facility. When using a central ejector, a heat exchanger is required to protect the steam generator from the test engine combustion gases, which increases the size of the facility. Conversely, an annular ejector does not require a heat exchanger, enabling a reduction in the equipment size. In a high-altitude test facility, unlike that of a central ejector, the secondary flow for an annular ejector is composed of supersonic combustion gases. In this study, we investigated the impact of the annular ejector shape and operational variables on the ejector’s entrainment ratio performance when the secondary flow is supersonic. The effects of the area ratio between the primary and secondary flows and the Mach number of the primary flow were examined using theoretical equations. Numerical analysis was employed to determine the impact of the divergence angle of the primary flow nozzle. Based on the study results, the divergence angle of the primary nozzle is recommended to be set between 9° and 19°. The findings of this study are expected to contribute to the design of annular ejector for high-altitude test facility.</p>