Based on the aerodynamic method, a performance calculation model for the variable operating conditions of injector was constructed. The mechanism of the momentum exchange in the transonic jetting flow in ejector was investigated, and the critical characteristics of the ejector were found based on analysis of ejection, pressure recovery and velocity characteristics. And a two-dimensional momentum exchange operator based on the area scale momentum exchange coefficient is proposed, and combining the velocity efficiency, momentum efficiency and pressure efficiency, the scale characteristics, boundary characteristics and shock dissipation characteristics in the transonic jetting flow were investigated. Research has found that the momentum exchange coefficient can perfectly reflect the momentum exchange characteristics inside ejector, determining the variation trend of suction flow rate. Under critical mode, the momentum exchange ability of ejectors is almost constant. The momentum exchange characteristics under critical modes have significant similarity and statistical invariancy.