The morphological and structural properties of plasticized spinning polyacrylonitrile (PAN) fibers during the stabilization process were investigated. The PAN fibers were subjected to stabilization treatment following a three-stage heating process. The PAN fibers were characterized by elemental analysis, Fourier transform infrared spectroscopy and X-ray diffraction; it was found that the stabilization degree of the PAN fibers is significantly higher than that of commercial PAN fibers. For plasticized spinning fibers, stabilization can occur in the crystalline region even at a low temperature of 180 °C; oxygen containing groups in the crystalline region generated during the plasticized spinning process may be responsible for this phenomenon. At a stabilization temperature of 230 °C, the cross-section of plasticized spinning fibers exhibits a large block stacking structure with obvious grooves among them, whereas a cross-section of the fibers shows a sheet-like divergence structure at 260 °C. The mechanical properties and density test results further indicate that 260 °C is a critical stabilization temperature for plasticized spinning fibers. Moreover, it is observed that a slight skin-core structure can be formed during the stabilization process.