Abstract: The physical processes of exhausting propellant and its migrating in space before the secondary startup of an engine in the high altitude and super-vacuum environment have been analyzed by using the vacuum physical theory. The assumptions used in the model and equations are given. An appropriate numerical method is developed to solve the equations and to conduct the computer simulation related to the physical process at the condition of vacuum. The contamination on satellite caused by the exhausting propellant is calculated. The unsteady process of concentration variation as well as the maximum contamination on satellite surface at different time is obtained. The theoretical criterion is provided for performing the propellant exhausting scheme desired.