Abstract: The ion thruster plume model was established and a numerical simulation using the particle-in-cell method was adopted to obtain charge-exchange(CEX) ions distribution in the plume. The full approximation storage-algebraic multigrid method is used to update the electric field. A three dimensional parallel particle simulation code has been developed by using the compute unified device architecture for graphic processor unit. The results show that CEX ions will go toward outside of the ion beam due to the expansion structure of the electric potential and some will go back to the upstream of the ion thruster. The ion number density is three to four order lower than that in the beam. The backflow current of CEX ions will be reduced as the electric temperature in the plume is reduced.