The effects of boric acid on the stabilization reactions of polyacrylonitrile (PAN) fibers were investigated by dipping fibers in boric acid aqueous solutions prior to drying process. The boron (B) contents in the PAN fibers are proportional to the concentrations of bath solution up to 2300 ppm when the boric acid concentration is 2.0 wt%. Methylene blue adsorption capacities of the doped PAN fibers decrease with increasing B. Differential scanning calorimetry spectra of the PAN fibers show the boric acid has a weak inhibition effect on the cyclization reactions and a significant hindering effect on the oxidation reactions. Combing with the elemental analysis and bulk densities of the stabilized fibers, it is considered that boric acid particles prevent oxygen from diffusing by occupying the space of the pores. However, by prolonging the stabilization period, the oxygen content in stabilized B-doped PAN fibers can be regulated to an appropriate value, and thus, carbon fibers owning excellent mechanical properties can be prepared. The B content in the resultant carbon fibers is nearly doubled compared to that of the relative PAN fibers.