This research investigates the Mach wave structure of the driving flow under off-design working conditions by both numerical and experimental methods. By adopting the method of characteristics as the simulation model, prediction of the driving flow regime inside an ejector is obtained. The simulation results are further validated by an experimental visualization method conducted using a Schlieren system. Through this investigation, the influence of Mach wave on the driving flow boundary development is discussed. The expansion wave from the nozzle exit increases the driving flow regime in the under expanded condition, which has a negative impact on ejector performance. The results show that the Mach wave should be considered when the ejector is operated under off-design working conditions. The results also demonstrate that an appropriate nozzle structure design was able to restrain the effect of the expansion wave, which improves ejector performance. The results are significant for achieving a comprehensive understanding of the mechanism of an ejector, as well as for the applications, such as ejection refrigeration cycles. (C) 2016 Elsevier Ltd. All rights reserved.