In this study, Computational Fluid Dynamics (CFD) technique is employed to investigate the effects of two important ejector geometry parameters: the primary Nozzle Exit Position (NXP) and the mixing section converging angle θ, on its performance. A CFD model is firstly established and calibrated by actual experimental data, and then used to create 95 different ejector geometries and tested under different working conditions. From 210 testing results, it is found that the optimum NXP is not only proportional to the mixing section throat diameter, but also increases as the primary flow pressure rises. On the other hand, the ejector performance is very sensitive to θ especially near the optimum working point. The entrainment ratio can vary as much as 26.6% by changing θ. A relatively bigger θ is required to better maximize the ejector performance when the primary flow pressure rises. The significance of the study is that these findings can be used to guide the adjustment of NXP and θ in order to obtain the best ejector system performance when the operating conditions are different from the on-design conditions.