Abstract: To study the starting characteristics of the variable cross-section supersonic-supersonic ejector with different structural and flow parameters, the two-dimensional Reynolds averaged Naiver-Stokes equations were employed to examine the different total pressure and total temperature ratios of primary and secondary flows in the mixing chamber of the ejector. The “start-up coefficient” was defined to test whether the supersonic-supersonic field was established. The results show that the critical total pressure ratio and total temperature ratio of ejector start-up first decreased and then increase with the increase of the contraction ratio (range 0.7−0.9). When the contraction ratio is 0.8, the optimal total pressure ratio is 5.88 and optimal total temperature ratio is 0.21. With fixed structural parameters, it is more difficult to start the supersonic-supersonic ejector when the total temperature ratio is higher. In the starting state of the supersonic-supersonic ejector, the chamber pressure is constant as the total temperature ratio and total pressure ratio vary. Also, compared with the ejector coefficient, the defined start-up coefficient can directly determine whether the supersonic-supersonic ejector has started or not, regardless of the specific working conditions.