Polyacrylonitrile (PAN) was cyclized by heating in an inert atmosphere and then dissolved in sulfuric acid and oxidized by nitric acid, resulting in product named OCPAN. The OCPAN was dissolved along with PAN and spun into fibers, getting modified PAN fiber (MPAN), to realize self-crosslinking and direct carbonization. The chemical structure of the OCPAN and its compatibility and co-carbonization effects with PAN were characterized through characterization methods of optical microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. The structure and properties of the resultant carbon fibers were investigated by scanning electron microscope and monofilament strength tester. The results show that ketone aromatic heterocycles are produced in OCPAN, which are able to promote the formation of a stable structure of carbon network and produce crosslinking with PAN molecules. The carbon yield of the MPAN fiber after direct carbonation under 900 A degrees C temperature in nitrogen is 23% higher than that of the PAN fiber without oxidation carbonized under the same condition, while the strength of the final carbon fiber is a bit lower than that derived from the PAN fiber through pre-oxidation. Therefore, the direct carbonation of PAN fiber is basically realized.