Abstract: This paper carries out a contamination study of a ZY satellite sensitivity surface. For the 20 N orbit-control thruster, the simulation program combines the axial symmetry domain division with three-dimensional DSMC method, with the multi-species in the plume being considered. The plume filed and the return flux on the sensitive surface are obtained. The simulation results show that the molecule of smaller molecular weight is easier to come into the return flux, and to cause species separation and non-equilibrium effect. The simulation model is verified by comparing the simulated results with the experimental data. This paper provides some guidance for the contamination simulation of sensitive surfaces.