Abstract: Presentanalysesontheionbeamofthegridsystemdonottakeaccounttheplasmadensityindischargerchamber.Ionbeamextractionwassimulatedbytheparticleincell(PIC)method.Theelectricfieldwascalculatedbythefinitedifferentialmethod(FDM),andtheionparticleswereacceleratedbytheleapfrogmethod.WithChinese20-centimeterXeionthruster,theelectricpotentialandionbeamdistributionsinthecenterandbrimapertureofgridsystemwereobtained.Theresultsshowthattheionbeamextractedbythecenteraperturehasbetterfocuspropertyandsmallerdivergenceanglethanthatofthebrimionbeam.However,thesaddlepointpotentialofthecenterapertureis24.8V,higherthanthatofthebrim.Hence,theplasmadensityindischargerchamberhasanimportanteffectontheionbeamextraction,andelectronbackstreamingphenomenatakeplaceonthecenterpositionofgridsystemfirstly.