A series of co-polyimide (PI) fibers containing phenylphosphine oxide (PPO) group were synthesized by incorporating the bis(4-aminophenoxy) phenyl phosphine oxide (DAPOPPO) monomer into the PI molecular chain followed by dry-jet wet spinning. The effects of DAPOPPO molar content on the atomic oxygen (AO) resistance of the fibers were investigated systematically. When the AO fluence increased from 0 atoms·cm?2 to 3.2 × 1020 atoms·cm?2, the mass loss of the fibers showed the dependence on DAPOPPO molar content in co-PI fibers. The PI fiber containing 40％ DAPOPPO showed lower mass loss compared to those containing 0％ and 20％DAPOPPO. At higher AO fluence, the higher DAPOPPO content gave rise to dense carpet-like surface of fibers. XPS results indicated that the passivated phosphate layer was deposited on the fiber surface when exposed to AO, which effectively prevented fiber from AO erosion. With the DAPOPPO content increasing from 0％ to 40％, the retentions of tensile strength and initial modulus for the fibers exhibited obvious growth from 44％ to 68％, and 59％ to 70％, after AO exposure with the fluence of 3.2 × 1020 atoms·cm?2. The excellent AO resistance benefits the fibers for application in low Earth orbit as flexible construction components.