The optical system is one of the main components of an ion thruster, which consists of electrically biased multi-aperture grids. The grid design is critical to the ion thruster operation since its transparency has an important influence on the thruster efficiency and thrust. To further optimize the optical system performance and evaluate effectively the efficiency of ion thruster, the optical transparency radial distribution of ion thruster is analyzed and discussed in experiment and simulation. The process of beam extraction is simulated by the particleincell-Monte Carlo collision (PIC-MCC) method, and the movement of the ions is investigated by the PIC method while the collisions of particles are handled by the MCC method. Then the interdependency among the transparency of screen grid, the accelerator grid, optics system and the number of ion extracted is analyzed. Taking into account the distribution of ion density at the exit of discharge chamber, the radial distribution of the screen grid transparency, accelerator grid transparency and optical system transparency are acquired. An experiment is performed to verify the simulation based derivation, indicating the good agreement between experimental and simulation results. The results show that the radial distribution of screen grid transparency increases gradually along the radial direction and has a good central axial symmetry, and its minimum value is located in the center of the thruster while the maximum value is near the margin region of screen gird. The radial distribution of accelerator grid transparency is opposite to that of the screen grid transparency, which decreases along the radial direction, and its maximum value is located at the axis of the thruster. The radial distribution of optical system transparency is the same as that of the screen grid transparency. And its minimum value is in the center of optics system, which indicates that the effect of accelerator grid transparency on the optical system transparency is little. In addition, the study also finds that the total optical transparency of ion thruster decreases slowly as the beam current increases. This work will provide a lot of support for the optimal design of ion thruster optics system.