During the pre- oxidation process of polyacrylonitrile (PAN) fibers, the behaviour of oxygen diffusion from the environment to the interior of the fibers leaded to differences in the oxygen content in the radial direction of the fibers, which resulted in different degrees of pre- oxidation reaction in the radial direction of the fibers and the formation of an inhomogeneous structure, whereas the subsequent carbonation and graphitization can not correct the defects brought about by the pre- oxidation, which ultimately affected the performance of carbon fibers. The effect of pressure on the degree of pre- oxidation and radial structure of PAN fibers was investigated by increasing the ambient pressure to provide the driving force for oxygen diffusion into the interior of the fibers, and at the same time, purely increasing the oxygen concentration also affected the diffusion of oxygen in the radial direction of the fibers, and the pre-oxygenated fibers prepared under the pressure and oxygen concentration were compared to analyze the mechanism of effect of different ways of changing the oxygen content on the pre-oxidation behaviour of the fibers. The results show that the degree of pre-oxidation reaction of the fibers increases from 30.65% to 35.49% when pre-oxidation is performed at 235 ℃ with a heat treatment time of 20 min, and the pressure is in the range of 10~20 kPa, which substantially promotes the pre-oxidation reaction of the fibers, while significantly weakening the skin-core structure of the fibers. Above 20 kPa, the effect of pressure on the pre-oxidation reaction and skin-core structure of the fibers is weaken. When the heat treatment time is increased to 40 min, the pressure mainly promotes an increase in the epoxidation index of the fibers and a higher degree of pre-oxidation. Compared to the oxygen concentration change, the degree of pre-oxidation of fibers at 10 kPa (30.65%) is greater than that of fibers at 23% (22.80%) at 235 ℃-20 min equivalent oxygen content, and the pressure has a greater effect on the degree of re-oxidation reaction of the fibers as well as the difference of radial structure.