It is crucial to enhance the entrainment performance of steam ejectors which operate under double-critical conditions to widen their application in industries. In this paper, combined auxiliary entrainment technical approach is proposed and corresponding geometric structure optimization has implemented systematically, for a better use of the inside low-pressure potential thus greatly improving the entrainment performance of the steam ejector under double-critical conditions. Moreover, a comprehensive analysis and discussion of the influence of the geometrical parameters on the auxiliary entrainment performance has been obtained from mass flow rate, pressure field and special internal flow characteristics. The results reveal that the combined auxiliary entrainment is the best choice for the given steam ejector operating under the double-critical conditions, and the optimum geometrical parameters of throat auxiliary entraining entrance are same as the designed condition and remain unchanged. For the diffuser auxiliary entraining entrance, the opening starting position X-s could be set at the entrance of the diffuser identically, the opening angle could be chosen as a common range R e from 75 degrees to 105 degrees. However, the optimum opening width d increases with the decrease of back pressure p(c). In general, there is an optimum geometrical parameters combination that the entrainment performance can achieve its maximum value for each p(c). The smaller the p(c), the bigger the entrainment ratio improvement, as large as 34.8% for p(c) of 32 kPa.