Abstract: Kr is one of the preferred propellants for the low cost ion thruster. The Kr plasma transport process was simulated with a 3D PIC method, and results were compared with those using the traditional Xe propellant. The influence of propellant type on optics performance was analyzed, such as the beam distribution, ions through rate and divergence angle loss. The results show that: the screen grid sheath of Kr plasma in discharge cavity is deeper than that of Xe plasma, and ions through rate of screen grid is higher; the axial velocity of Kr ion is bigger than that of Xe ion near the accelerator grid hole, which means the radial displacement distance is relatively small and the accelerator grid current caused by ion impact decreases; the divergence angle loss of Kr propellant is the same as that of Xe propellant, but the operation current is higher; the thrust of Kr decreases about 20% compared with that of Xe propellant for the same beam current and accelerating potential. The results of the study have important significance to optimize the propellant for ion thruster.