Abstract: Focusing and extraction efficiency have a significant effect on electron impact ion source（EI）.Themotion of the individual EI of a MiniatureMagnetic Sector Mass Spectrometerwas simulated by means of the ion optics simulation software package,SIMION-3D8.0,to analyze its position and velocity.Theinfluence on its characteristics,including the focusing and velocity divergence of the emitted ion beam of the EI,through the EI parameters was evaluated.The simulated results show that If S slit and αslit are respectively applied negative high bias potential,the ion beam can focuses two times in a individual motive process.We suggest that fine electron impact ion source can be designed,fabricated,and integrated with themass analysis by optimizing the EI parameters.