AbstractWood-based carbon fiber derived from liquefied wood has the disadvantages of low mechanical strength and unstable performance. To improve its mechanical properties, wood-based carbon fiber precursors were impregnated with 5 wt.% and 8 wt.% boric acid solutions for 1 h and then carbonized at 900 °C for 1 h. The effect of boron content on fiber tensile strength and microstructure was investigated through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The tensile strength of wood-based carbon fibers impregnated with 5 wt.% boric acid reached 0.468 GPa, an increase of 44.89% compared with the untreated samples. In addition, the amount of graphitoidal crystal and the degree of graphitization increased with the boric impregnation. Furthermore, boron in the form of a boron-carbon solid solution in the graphene layer of carbon fibers effectively improved the tensile strength of wood-based carbon fibers.