Abstract: Air breathing electric propulsion system collects, compresses, ionizes and accelerates the environmental atmosphere to generate thrust compensating the drag on the spacecraft in the very low Earth orbit. This technology is one kind of space resource in-situ utilizing technology, it can reduce the launching cost of the spacecraft, increase the spacecraft’s lifetime and make full use of the very low orbit resource, it has broad application prospects. The environmental gases in the very low Earth orbit mainly include N₂, atomic oxygen （AO） and O₂, because of the relatively high reactiveness of the AO, when it is collected and compressed the number of the AO will decrease affecting the quantity of the AO entering the electric thruster then affecting the performance of the electric thruster. In order to determine the gas composition entering the electric thruster, AO flux decaying experiment was carried on the ground. In this paper, the changing rule of the AO’s flux with the distance was examined with environmental gas pressure being 0.05 Pa, 0.5 Pa and 1.0 Pa respectively. The obtained results indicate that the higher the pressure of the environmental gas is, the smaller the AO flux is. Under the same three pressure, in the range of 20 cm, the AO flux decreases by a factor of 20. The passive intake device of the air breathing electric propulsion system can compress the environmental gas more than 200 times, turbo-molecular pump can compress the gas from the passive intake further. The pressure of the gas is about the order of 1Pa. The working matter entering the electric thruster is approximately the mixture of N₂ and O₂. Some protective measures should be taken against the AO of some ratio produced in the discharge chamber.