Abstract: The chemical structure changes of three polyacrylonitrile fibres used for making carbon fibres were measured continuously during low temperature (350℃) pyrolysis in air atmosphere by the in-situ Fourier transform infrared spectroscopic method. From the information of the qualitative and quantitative spectra, it was obtained that the dehyrogenation and oxidation reactions proceeded mainly after the cyclization reaction for the British fibre, the dehyrogenation reaction occurred mainly after cyclization and oxidation reactions for the Jilin fibre, but the cyclization and dehyrogenation reactions occurred mainly before the oxidation reaction for the Lanzhou fibre. It was shown that the maximum length of cycloforming sequence for the British fibre was obtained at 260℃, for the Lanzhou fibre at 300℃, and for the Jilin fibre at 320℃. The length of cycloforming sequence of the Lanzhou fibre was the shortest. Wherefore, the results are valuable, and it may be expected that high performance PAN-carbon fibres can be obtained by improving the operation conditions of carbonization.