The invention relates to a method for preparing cyclized PAN (polyacrylonitrile) fiber by using aromatic cyclization of a PAN spinning liquid. The method comprises the following steps of dissolving PAN into a solvent, so as to obtain a PAN/solvent spinning solution; placing into a sealed reaction kettle, and introducing CO2 (carbon dioxide) to completely replace air; heating, introducing the CO2 again, and performing the aromatic cyclization with phased heating under the supercritical condition; relieving pressure to normal pressure, so as to obtain the aromatic-cyclized PAN/solvent spinning liquid; spinning, and stretching, so as to obtain the cyclized PAN fiber. The method has the advantages that the economic and environment-friendly effects are realized, the reaction is controllable, and the reaction time is short; the adding of other solvents is not needed, and the pollution and the usage of solvent are certainly reduced; the cyclization temperature of the PAN is greatly reduced, the energy consumption in the heating process is decreased, the experiment procedure is simplified, the labor and material are reduced, and the larger industrial application value is realized.