Abstract: To clarify the origin of heat accumulation in large-tow polyacrylonitrile (PAN)-based carbon fiber precursors during thermo-oxidative stabilization, this work investigated the effects of tow size, spreading state and reaction-heat source on local center-temperature response and radial structural inhomogeneity. In-situ local center-temperature monitoring, skin-core structure observation, Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC) were used. Composite bundles composed of PAN precursors and fully stabilized fibers were further designed to evaluate, in a qualitative and semi-quantitative manner, the apparent contributions of environmental heat transfer, heat transferred from outer reacting fibers and heat released by inner reacting fibers. The results show that 12 K and 24 K bundles reacted steadily at 242℃, whereas 48 K and 72 K bundles exhibited obvious heat accumulation and densely packed 96 K bundles tended to undergo thermal runaway. Increasing the spreading width reduced the local center peak temperature of 96 K bundles, indicating an overall improvement in heat dissipation. FT-IR results show that surface fibers reacted faster at the early stage, while the central fibers accelerated later because of local heat accumulation; for 96 K bundles, the reaction degree of the central fibers exceeded that of the surface fibers at about 6 min. In the composite-bundle model, each additional 24 K inner PAN fibers increased the local center peak temperature of the 96 K composite bundle by about 2.9℃. When the average local center heating rate within the first 1 min was used to describe the initial environmental heat-transfer efficiency, each additional 24 K fibers reduced this value by about 8.9℃/min. These results indicate that heat accumulation in large-tow PAN precursor bundles mainly originates from the combined effect of continuous heat release from inner PAN fibers and hindered heat dissipation within the bundle.