Abstract: Pre-oxidation is attributed to a gas-solid state reaction where oxygen diffuses from the fiber surface to its core along the radial direction. This process is inherently slow, time-consuming, and energy-intensive. Consequently, elucidating the reaction and regulation mechanisms of pre-oxidation has emerged as a study focus. The heating rate, one of the most significant process parameters, significantly influences oxygen infiltration and pre-oxidation reaction. In this work, the effect of the heating rate on the pre-oxidation process of pitch-based carbon fibers was investigated using diverse characterization methods, including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and thermogravimetric analysis (TGA). These methods provide a comprehensive analysis of the microscopic changes occurring during the pre-oxidation process in the aspects of molecular composition evolution, oxygen permeation, and thermal reaction properties. The results show that a heating rate of 0.5 ℃/min ensures adequate pre-oxidation of pitch fibers. Oxygen permeation promotes oxidation, condensation, dehydrogenation, and decomposition reactions in the aromatic and polycyclic structures within the three-dimensional pre-oxidation fibers, resulting in cross-linking and bridging structures. This transformation renders the fiber structure more ordered, further influencing its thermal stability. As the heating rate increases, the oxygen yield in pre-oxidation fibers decreases, and the number of oxygen-containing functional groups decreases, making the adsorption vibration signal of C—O and C O weaker. It suggests that higher heating rates reduce pre-oxidation efficiency. The content of oxygen-containing functional groups such as C O, C—O—C, and O—C O increases significantly as the heating rate is 0.5 ℃/min. When the heating rate exceeds 0.5 ℃/min, the gradient of oxygen concentration between the fiber surface and center is more pronounced, leading to a greater disparity in oxygen concentration. This indicates that excessive heating rates hinder oxygen permeation into the fiber interior, decreasing the introduction of oxygen atoms and causing insufficient oxidation. Consequently, small molecules are more likely to be released in carbonization, compromising the stability of the carbon fibers. Carbon fibers produced at a heating rate of 0.5 ℃/min demonstrate a smooth surface and well-defined fiber morphology.