A Si/P flame retardant was prepared using tetraethyl orthosilicate (TEOS) and phosphoric acid (H3PO4). Original cotton fabric, treated fabric and its charcoal were examined by FT-IR, SEM, and EDS. Results showed that the TEOS/H3PO4 flame retardant has little affect on the cotton chemical structure and the surface morphology of cotton fiber. Main absorption peaks of the charcoal in the FT-IR are attributed to the P-O-C and the Si-O-Si bonds. And the surface of the charcoal was covered with white particles. EDS illustrates Si-O-Si bonds form on the surface and exhibit white particles in the SEM image.