Steam ejectors are regarded as a promising energy-saving technology, which have been widely used in many industries. The aim of this paper is to optimize the geometrical parameters of the auxiliary entraining entrance that has a significant influence on the entrainment performance. In this study, the developed auxiliary entrainment and structure optimization make good use of the low-pressure potential inside the throat for performance improvement. Moreover, the influence of auxiliary entraining entrance geometric parameters on internal flow fields and mass flow rate have been obtained numerically and analyzed carefully, and the special internal flow characteristics are also presented and discussed in detail to emphasize the role of auxiliary entraining entrance geometric parameters in determining entrainment performance. The results reveal that the major influence factors of entrainment performance are the effective suction pressure area, the near-wall mixed steam velocity and the negative affect on main entraining entrance. And there are an optimum geometrical parameters combination of the auxiliary entraining entrance, whose opening center position is 119mm, opening width is 5 mm and opening angle is 90 degrees for the given ejector and operation parameters, with a maximum entrainment ratio improvement of 3.80% comparing with that of the conventional designed steam ejector. Furthermore, the auxiliary entraining entrance can be designed in an appropriate range.