The boron-containing phenol-formaldehyde resin-based ceramifying composites that used muscovite mica and glass frits loaded boron-containing phenol-formaldehyde resin (BPF) as matrix, high silica fiberglass fabric as reinforcements, were pyrolyzed into ceramic gradually in the air. Glass frits were fused into liquid phase and spread to the surface to make muscovite mica form compact mullite ceramic shell. The shell restrained oxygen into the internal effectively in order to reduce the thermo-oxidative degradation of BPF resin. X-ray diffraction analysis (XRD) showed that aluminium borate (Al8B4O33) and mullite (Al6Si2O13) crystalline phases after pyrolysis. SEM demonstrated the ceramifying progress of the microstructure of the composites, and EDS analyzed the micro-chemical composition.