Abstract: The applicability, advantages and disadvantages of quasi-neutral assumption, Boltzmann distribution assumption, and particle model of electrons in the electric thruster flow simulation are analyzed. Based on the analysis, a new method: electron drift-diffusion approximation is presented, which is used to simulate the plasma movement in optical system of ion thruster, and its feasibility is verified in optical system simulation. Simulation results show the plasma potential distribution, ion phase space distribution and the electron density distribution is consistent with the results of the Boltzmann distribution assumption, and the feasibility of this new method is verified.