Phosphate-containing flame-retardant polymers were synthesized via a partial-substitution reaction of poly(vinyl acetate) with stearylphosphonic chloride (SPPC) and diethylchlorophosphate in various substitution ratios. SPPC had been previously synthesized from phosphonic dichloride and stearyl alcohol. The synthesized SPPC and the flame-retardant polymer products mere characterized by Fourier transform infrared (FTIR) spectra analysis. The quantitative analysis of phosphorus in the polymer products was carried out by EDS (Energy Dispersion Spectrummeter) analysis. The real substitution ratios of the polymer products were estimated by the atomic number ratio of carbon and phosphate in products from EDS data. Variations in the glass-transition temperature and thermal-decomposition temperature of the products were investigated. The char of polymer products produced after thermogravimetric analysis (TGA) was also analyzed by FTIR, and the results show that different phosphate substitution leads to different char developments during the heating of TGA.