Abstract: In order to save energy consumption and time of the pre-oxidation process and optimize the performance of polyacrylonitrile(PAN) pre-oxidized fiber, PAN precursor could cyclize in advance with modification with H₂O₂. The unmodified and modified PAN precursor treated at different temperatures was analyzed by FTIR and XPS, et al. The results show that the modification with H₂O₂ aqueous solution at 60℃ induces nitrile of the PAN precursor to cyclize and terminal ring structure tautomerizes, hence it brings about imine and quasi-aromatic primary amine. With the increase of the modification temperature, the content and conjugation degree of the imine of modified PAN precursor is promoted. In the process of the simulated stabilization, quasi-aromatic primary amine of the modified PAN precursor could induce adjacent nitrile to cyclize at lower temperature. The modified PAN precursor with NH₃H₂O as additives experiences the same pre-oxidation process as the unmodified PAN precursor. The former can reach suited pre-oxidation degree within a short time and the radial structure of PAN pre-oxidized fiber becomes more uniform, so that the PAN pre-oxidized fiber which has higher thermostability can be obtained.