With increasing awareness of environmental concerns, ejector refrigeration system which utilizes the low-grade thermal energy, such as industrial waste heat and solar energy, becomes increasingly popular. Nevertheless, due to the relatively low coefficient of performance of the ejector refrigeration system, it is necessary to improve its efficiency. As the core component of this system, the effectiveness of the steam ejector is the key factor to improve the overall performance. The complicated flow structure and mixing process within the steam ejector lead to a relatively low entrainment ratio. Many numerical studies based on the ideal gas assumption have been carried out to investigate the flow structure and impact factors of the performance for the steam ejector. Considering the actual experiment cases, spontaneously condensing phenomenon happened in the whole working process, while most simulations focus on only the nozzle part of the steam ejector leading to a significant deviation between numerical and experimental results. In this thesis study, a wet steam model was proposed to investigate the working process and the condensation effect occurred within the steam ejector of the entire system. Compared with simulation results generated by the ideal gas model, the present results utilizing wet steam model show good agreement with the referenced experimental data and improve the accuracy of the numerical prediction on the axial pressure distribution. Furthermore, the geometry of the mixing chamber has been studied and the optimized size of the steam ejector was discovered, which improves the performance of the steam ejector with the same operating conditions. Moreover, the influence of the primary flow pressure on the condensation effect was studied. Increasing primary flow pressure did not significantly affect the condensation of droplets and the occurrence of the shock wave is shifted further within the system. Finally, the influence of the wetness of inlet flows on the performance of steam ejector is also investigated. Under numerical conditions, it is found that reducing the wetness of secondary flows has a positive impact on improving the entrainment ratio of the steam ejector.