Abstract: Due to its high specific impulse, long lifetime and continuously adjustable thrust, the ion thruster has been applied to the satellite position maintenance and orbit transfer. The ion thruster has become the best choice of the dynamic system for position maintenance and orbit transfer missions. But its thrust is so small that it is asked for working thousands or even tens of thousands of hours, which will be more for the deep space missions. Variable aperture, a new concept of the ion optics, could improve the plasma flatness and increase the lifetime. By artificially decreasing the aperture diameter of the screen grid, the plasma density of the grid center will be reduced and it could make the thruster lifetime longer. This paper establishes an ion optics numerical model. Two adjacent apertures with different diameters are chosen and their interactions are taken into account. A simulation method is used to simulate the electric potential distribution and the beam ions density distribution, where the ions from two apertures are extracted simultaneously. This paper aims at investigating the effects of the variable apertures on the perveance characteristics and the changes of the electric field. The simulation results show that the screen grid diameter could not have an influence on the electric potential distribution and the beam ion perveance when the plasma density upstream of the screen grid is constant. However, the screen grid diameter directly impacts on the ion optics performance and lifetime. If the diameter of the screen grid is decreased by 5.2%, the average beam current extracted is reduced by 2.7% and the plasma density upstream of the screen grid is increased by 1.2%. The CEX ion density is increased from 0.19 to 0.22, about 13.6% if the screen grid diameter increases from 32.36 to 34.26, and the maximum erosion depth of the accelerator grid increases by 11.1%.