Developing sustainable bio-based thermoset resins with exceptional heat resistance and flame retardancy remains a critical challenge. This study synthesized a novel guaiacol-derived benzoxazine-containing phthalonitrile resin (GB-PN) and prepared its polymer Poly(GB-PN). The curing behavior, polymerization mechanism, and flame-retardant mechanism were systematically investigated. The optimal curing temperature was determined via curing kinetic parameters. Poly(GB-PN)-360 °C exhibited outstanding thermal stability, with Td5 and Td10 values of 461 °C and 507 °C, respectively, and a high char residue yield (Yc) of 66.2 %. Moreover, Poly(GB-PN)-360 °C exhibited significantly superior flame retardancy compared to analogous thermosetting resins, evidenced by a limiting oxygen index (LOI) of 43.98 %, a heat release capacity (HRC) of 15.5 J/(g·K), and a total heat release (THR) of 1.4 kJ/g. Characterization by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman spectroscopy, and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) confirmed the residual char mechanism of Poly(GB-PN)-360 °C and elucidated its pyrolysis pathways and char layer composition. This work provides an effective strategy and theoretical insights into char formation for developing thermosetting resins that integrate superior thermal resistance and flame-retardant properties.