Abstract: In each leak testing stage for a satellite, the propulsion system must be charged with gas. For the satellite propulsion system, the process of gas charging must be paid great attention to for an effective control of redundant substance. This paper analyzes the factors influencing the control of redundant substances in the gas charging process during the leak testing of the propulsion system based on a large number of tests and the data obtained from the tests, and the methods of controlling the redundant substances. The result can be applied in the propulsion system of various kinds of spacecraft to control the redundant substances.