In this work, we proposed a novel phosphorus-free and halogen-free strategy to prepare flame retardant polyacrylonitrile fibers (FR-PAN). First, PAN fibers were modified with hydroxylamine hydrochloride (HA) to introduce amidoxime groups in PAN fibers (A-PAN), then the A-PAN samples were immersed in ferric chloride solution to chelate Fe³⁺ to obtain FR-PAN. The micro calorimeter (MCC) test showed that the peak heat release rate (PHRR) and total heat release (THR) of FR-PAN were reduced by 72.9%, and 86.9% respectively. Meanwhile, the limiting oxygen index (LOI) value of FR-PAN increased from 17.0% to 36.7%. TG results demonstrated that amidoxime groups promoted the cyclization and dehydrogenation of PAN in advance. Meanwhile, the released nonflammable NH₃ effectively diluted the emission of combustible gases, which acted as a flame retardant role in gas-phase. Fe³⁺ had synergistic catalytic char formation effect, resulting in a highly dense char layer containing Fe₂O₃, which isolated oxygen, significantly reduced the emission of toxic gases. This work endowed PAN fibers with good flame retardancy and great potential application in the field of functional fibers.