Phosphorus oxychloride (POCl3) was selected to chemically modify polyvinyl alcohol (PVA) films to prepare phosphorylated PVA films (PPVA), and the structure and properties of the resulting films were characterised using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetry (TG), scanning electron microscope (SEM). The results showed that some of the hydroxyl groups on the PVA polymer chain were phosphorylated. With the increase in phosphorus content, the Tg of PPVA film gradually decreased, and the crystallization ability gradually weakened, the tensile strength and elongation at break of PPVA membrane gradually decreased, while the membrane thermal stability, flame retardant properties and water resistance were improved. When the phosphorus mass fraction was 1.44% (PPVA-2), the flame retardant grade of the modified film reached the UL 94 V-0 grade, and the limiting oxygen index (LOI) reached 26.1%. The residual carbon rate of PPVA film gradually increased with the increase of phosphorus content, when the phosphorus mass fraction was 4.14% (PPVA-5), the residual carbon rate reached 33.0%, much higher than the residual carbon rate of PVA film at 600℃ of 5.7%, indicating an improvement of 478.9%. After phosphating treatment, PVA changed the chemical structure of its polymer chain, and the structure of the aggregation state was also altered, so that the membrane thermal stability, flame retardant properties and water resistance, etc. were improved.