At present, traditional drainage gas recovery technology is not suitable for low-pressure gas wells. In this work, a new type of ejector drainage gas recovery technology was proposed to solve this problem. High-pressure gas was used as a driving fluid to prevent secondary contamination. The problem of low efficiency was solved by forming a composite process with the foam scrubbing, and the ejector was the core of the technology. The aim of this study was to investigate the ejector flow characteristics for gas flow field by Computational Fluid Dynamics (CFD), the entrainment ratio was selected as the performance index, and the influence of ejector structure size and operation condition was obtained. The results showed that the ejection process both of pumping action and shear acceleration action existed in ejection process, and the shear acceleration action only made the ejection fluid to run up to sonic speed. The mass flow rate of ejection fluid was controlled by area and speed together, if the flow area was steady, and the gas speed could not increase in any condition, so that the flow state was in chocking phenomenon. In the chocking condition, the performance of ejector was determined by pumping action. Furthermore, the chocking condition retarded with the increasing of diffused segment conical degree and area ration of the ejector nozzle. In addition, the shear acceleration action played the major role if the flow was not in chocking condition. Operation parameters had an impact on shock wave intensity and ejector flow state, the increasing of operation pressure and the decreasing of outlet pressure would enhance entrainment ratio and cause the chocking condition occur.