Abstract: As a simple and stable vacuum acquisition device, the internal flow process of the steam ejector is very complicated. At present, the research on the steam ejector has been relatively perfect, but the research on the expansion state of the steam ejector is rarely involved. To further improve the classification of the expansion state, the internal flow behavior of the steam ejector under different expansion states and its influence on the performance of the steam ejector are studied. In this paper, the CFD method is used as the main research method to study the flow field structure of different nozzle exit states and its influence on the performance of the steam ejector. The results show that the nozzle exit state can be divided into under-expanded state, full-expanded state, over-expanded state and non-Laval nozzle state, and the change of the secondary fluid pressure has a greater influence on the wall shear stress of the ejector than that of the nozzle. While taking into account the pumping capacity, efficiency, exhaust capacity and vacuum capacity of the steam ejector, the steam ejector can obtain a higher entrainment ratio and compression ratio in the full-expanded zone and over-expanded zone, and there is an optimal working condition in the over-expanded zone. In addition, the full-expanded zone is only a point value.