To eliminate the erosion in the apertures at the edge of the decelerate grid of a dual-mode ion thruster three-grid optical system, a new method of reducing the diameter of the apertures in the outer region of the screen grid is proposed. In order to investigate the method of influencing the discharge loss and the uniformity of the beam current density and also reducing the aperture erosion region and erosion rate of the decelerate grid, two kinds of three-grid optical systems are designed and fabricated with the same material and physical parameters except the diameter of the apertures in the outer region of the screen grid. The first kind is that the diameter of the aperture in the outer region of the screen grid is equal to that in middle region of the screen grid, it is designated as the same aperture diameter grid optical system. The second kind of three-grid optical system is that the diameter of screen apertures whose center distance from the center of grid is larger than 0.95 times the beam radius, is reduced by 26% and therefore the physical transparency of optical system is reduced by 8.8%, it is designated as the small aperture diameter grid optical system. The comparison between the same aperture diameter grid optical system and small aperture diameter grid optical system is performed by assembling them into a 30-cm-diameter dual-mode ion thruster, and the ion thruster performance test, beam flatness test and 600-h-endurance test are conducted in the two typical operation modes, namely the low thrust-high specific impulse mode and large thrust-high power mode. A comparison of small aperture diameter grid optical system with the same aperture diameter grid optical system shows that the discharge loss of the 30-cm dual-mode ion thruster in the low thrust-high specific impulse mode and large thrust-high power mode are reduced by 10% and 21% respectively, the beam flatness is decreased by 3% and 10% respectively, the number of rows of the apertures which are sputtered by beam ions at the edge of the decelerate grid is reduced from 5 to 1. In addition, the erosion rate is significantly reduced and the erosion phenomenon disappears after 900-h-long-duration operation. These results signify that reducing the screen grid aperture diameter in the outer region is an effective method to eliminate aperture erosion at the edge of the decelerate grid of a dual-mode ion thruster three-grid optical system and that this method will not reduce the efficiency of the thruster, but cause the beam current uniformity to worsen.