In this paper, a CFD model calibrated by the experimental results from initial designed ejector is used to evaluate the influence of 6 key geometry parameters on the performance (entrainment ratio) of an air-cooled ejector cooling system and, consequently, to find the best design parameters. A new ejector according to the findings from the CFD simulation is then designed and used at the same air-cooled ejector system to verify the simulation results. From both simulation and testing results, we find that: 1) the optimal area ratio, the ratio of primary nozzle exist position and length of constant-area mixing section to primary nozzle diameter are lower than those of water-cooled ejector systems; 2) the optimal converging angle of constant-pressure mixing section and the ratio of primary nozzle exit position and length of constant-area mixing section to the diameter of constant-area mixing section are very close to those of water-cooled ejector systems; 3) substantial performance improvement can be achieved by using the new parameters in the ejector design.