Abstract: The rational choice of mixing chamber structure is important to improve the performance of vapor ejector systems. The critical performance differences between conical-cylindrical mixing chamber and cylindrical mixing chamber ejectors were investigated with a nozzle exit position (NXP) and area ratio (Ar) adjustable ejector, and a comprehensive comparison of the two types of ejectors was made by introducing thermodynamic perfection and entropy generation rate models. The results demonstrated that compared with the performance of the cylindrical mixing chamber ejector (CME) at the optimal nozzle exit position, the equal area ratio conical-cylindrical mixing chamber ejector (CCME) exists a NXP range where both the critical entrainment ratio and critical condensing pressure are larger (i.e., multi-objective optimization is achieved). The mechanism of this phenomenon is the use of CCM can reduce the momentum loss of the mixed flow and increase the effective flow cross-sectional area of the secondary vapor. The NXP range can be widened by appropriately reducing the Ar of the CCME. Besides, the thermodynamic perfection of the CCME is more advantageous in the range of the experimental NXP than CME. When reducing the Ar of the CCME, the thermodynamic perfection of the CCME can be further improved within a certain range of NXP, and the thermodynamic reason is that the entropy production rate inside the CCME is further reduced. The results provide a new idea for the multi-objective optimization of the critical performance for variable geometry ejectors.