The relationship between the degree of ring formation and the overall properties of the pre-oxidized fibre was studied by treating PAN precursor with KClO₃/H₂SO₄ solution and different pre-oxidation techniques. PFs are polar materials and the dielectric constant increases after solution treatment. The bulk density of Cl-MSFs is always higher than that of MSFs and CSFs in the same temperature zone. The bulk density of Cl-MSFs5 is 1.401 g/cm³, which is 1.6% higher than that of MSFs5. The EOR value of Cl-MSFs5 is 0.767, which is 11.97% higher than the value of MSFs5. The modified polyacrylonitrile precursor showed a high degree of cyclization during microwave heat treatment. The temperature can be reduced by about 40 °C by modifying and microwave heat treatment to achieve the same level of cyclization. Cl-MSFs5 has a higher O content than MSFs5. The microscopic morphology shows that the number of "grooves" and fibre diameter of Cl-MSFs5 is significantly smaller than that of MSFs5, indicating that it has a higher degree of stability. There is no Cl element on the surface of Cl-MSFs5, which indicates that the structure of the pre-oxidized fibre is not affected after the pre-oxidation treatment. In addition, the structural transformation of KClO₃-modified PFs during microwave pre-oxidation is proposed. The cyclization reaction occurs first, followed by dehydrogenation and oxidation reactions. In the carbon fibre production process, the pre-oxidation stage takes a long time and consumes a lot of energy. The new composite treatment technology used in this study can effectively reduce the treatment temperature and shorten the treatment time in the pre-oxidation stage, which provides a reference for the development of low-cost and efficient pre-oxidation process.