During the pre‐oxidation process, skin‐core structure is easily formed in the fibers due to the competitive mechanism of oxygen capture and oxygen diffusion, which would degrade the tensile properties of the resulting carbon fibers. To prepare high‐performance carbon fibers, it is vital to optimize the radial structure of polyacrylonitrile (PAN) pre‐oxidized fibers. Methods commonly used to solve the skin‐core structure would result in low productivity and comprehensive performance. Here, we examine the effects of pressure on the chemical structure and the evolution of radial structure in PAN pre‐oxidized fibers under 225°C in the air, by using 13C Solid‐state NMR analysis (13C‐NMR), Optical density test (OD), and Thermogravimetric Analysis (TG). It was found out that under the positive pressure of 30 kPa, the reaction area expands to the core of the fibers with the shell thickness increasing from 88.11% under normal pressure to 98.67%, and the radial structure homogeneity is effectively improved. As the pressure increases above 22 kPa, oxygen diffusion is accelerated, allowing for breaking the barrier of the fiber surface capture and promoting the oxygen catalytic cyclization reaction in the core. Consequently, PAN pre‐oxidized fibers prepared under positive pressure have a more uniform radial structures and higher thermal stability. The carbonization yield at 800°C of the fibers prepared at 30 kPa is 13.10% higher than that of the fibers prepared at atmospheric pressure.