Based on the particle-in-cell (PIC) method, a two-dimensional numerical schemewas developed to investigate the ion beam extraction phenomena through the ion thruster optics.According to the calculated results, the plasma sheath upstream of the screen grid, the electricfield in the calculation domain, and the ion and electron spatial distributions are obtained fordifferent accelerator grid voltages. The results indicate that the accelerator grid voltage affectsthe plasma sheath upstream of the screen grid significantly. It is found that a moderate acceleratorgrid voltage results in an ion optical performance better than either a higher or lower voltage, froma point of ion extraction from the discharge chamber and erosion mitigation of the accelerator griddue to the direct ion impingement. Based on the particle-in-cell (PIC) method, a two-dimensional numerical schemewas developed to investigate the ion beam extraction phenomena through the ion thruster optics. According to the calculated results, the plasma sheath upstream of the screen grid, the electricfield in the calculation domain, and the ion and electron spatial distributions are obtained for dififferent accelerator grid voltages. The results that that the accelerator grid voltage affectsthe plasma sheath upstream of the screen grid significantly. It is found that a moderate acceleratorgrid voltage results in an ion optical performance better than either a higher or lower voltage, froma point of ion extraction from the discharge chamber and erosion mitigation of the accelerator griddue to direct ion impingement.