In the present paper, two sulfur-containing benzoxazine monomers were synthesized using 2-thiophenemethylamine as an amine source. The polymerization behaviors of the two monomers were compared with those of aniline and furfurylamine using differential scanning calorimetry (DSC), which revealed that the polymerization temperatures of the synthesized monomers were between those of aniline and furfurylamine. Thermogravimetric analysis (TGA) confirmed that the polybenzoxazines formed from the synthesized monomers had outstanding thermal stability. Pyrolysis–gas chromatography–mass spectrometry (Py-GC–MS) and TGA–Fourier-transform infrared spectroscopy (TGA-FTIR) were used to monitor the decomposition processes of the polymers. They allowed us to analyze the crosslinked structures of the polybenzoxazines and revealed the probable cause of their thermal stability. We found that the reliability of the thioether bonds and the di-substituted structures formed in the polybenzoxazines based on furfurylamine and thiophenemethylamine ensured the stability of the crosslinked structures. The methyl-substituted ring structures exposed by fracture react with the ammonia released at higher temperatures to form CN groups. Therefore, nitrogen is retained in the crosslinked structures and the thermal stability of the polymers is improved. Compared with common halogen- or phosphorus-containing flame-retardant polymers, the newly developed sulfur-containing polybenzoxazines are more environmentally friendly flame-retardant polymers.