Supersonic ejector is an important component of the high altitude test facility. Evacuating the test cell before engine start by the ejector can arrest the rocket exhaust leak into the cell, thereby prevent the potential exhaust separation in the rocket nozzle. Meanwhile, the start phase of the ejector is the most critical condition for ejector operation, and an ejector designed improperly will result in an unacceptable high starting pressure and inability to be started of the ejector. In order to investigate the starting performances under this condition, a test facility for small scale supersonic air ejector is constructed. By using pneumatic method and schlieren technology, the flow field is studied when the ejector on utmost starting pressure ratio. The result shows that the utmost starting pressure goes down with the mixing-room contraction ratio. The low mixing-room contraction ratio will cause the ejector unable to be started as well as insufficient ejecting pressure. It is observed that the chamber pressure will go up monotonously with the ejecting pressure when the inability of the ejector is induced by low mixing-room contraction ratio. In contrast, if the ejector unstart is induced by insufficient ejecting pressure, the chamber pressure will go down with the ejecting pressure.