Ocean thermal energy has attracted much attention for its huge potential resources. It's proved that an ocean thermal energy conversion (OTEC) cycle system using ammonia ejectors is beneficial to improve the cycle efficiency. Thus, the performance of an ammonia ejector is studied numerically in this paper. Effects of working conditions which occurred inside the ammonia ejector, like primary flow pressure (1.3‐2.1 MPa), secondary flow pressure (0.45‐0.65 MPa), and back pressure (0.5‐1.05 MPa), on the mixing process and flow phenomenon, are discussed in detail. The results demonstrate that the entrainment ratio of ammonia ejector ranging from 0.35‐0.65 increases with decreasing primary flow pressure at the critical working mode, while the reverse is true for the critical back pressure. Two small vortexes are formed near the ejector wall due to the boundary layer separation when the ejector works at the subcritical or critical mode. These vortexes reduce the effective area and flow rate of the secondary flow. The Ma contours are nearly the same in the nozzle and mixing zone for the critical and subcritical mode, but distributions of Ma vary dramatically in the diffuser corresponding to the variation of secondary series of oblique shock.