To further meet the plasma heating requirements of EAST (experimental advanced superconducting tokamak) device, NBI (neutral beam injection) system requires higher beam power. Accordingly, the EAST NBI laboratory has developed a 120 keV accelerator for this purpose. The analysis of the whole accelerator needs to be carried out from various perspectives such as beam optics, insulation support, active cooling and materials. Taking into account factors such as plasma parameters, electrostatic lens, voltage resistance between grids, and assembly errors, the numerical simulation program was utilized to beam optics analysis for the slit tetrode ion source accelerator. Beam trajectory, electric field strength distribution, and beam divergence angle were investigated and optimized to preliminarily determine the grid parameters of the new multi-slit accelerator. The accelerator obtains the beam with the minimum divergence angle of 0.6° in the vertical direction with perveance of 1.52 μp, meeting the design criteria for beam current intensity of 60 A and divergence angle below 1° of the ion source.