Using high degree of polymerization ammonium polyphosphate (APP) as a flame retardant and polyvinyl alcohol (PVA) as a film-forming framework material, a PVA/APP modified separator was prepared through phase conversion method and used in lithium batteries. Characterize the mechanical strength, wettability, thermal stability, microstructure, and electrochemical performance of the modified separator. Explore the effect of different APP additions on the performance of the separator and evaluate the cycling rate performance of its assembled battery. The results showed that when the PVA mass fraction was 10% and the APP addition was 8%, the modified separator had excellent electrolyte wettability and thermal stability, with a liquid absorption rate of 215% and almost no shrinkage at 200 ℃; The tensile strength reaches 47.4 MPa. The battery was assembled using a modified separator and cycled 50 times under a discharge condition of 0.1 C. The specific discharge capacity was 143.2 mAh/g, and the Coulombic efficiency was higher than 97%. The capacity retention rate reached 95.1%, while the capacity retention rate of the battery assembled with a commercial lithium battery separator was only 82.8%. The modified separator has flame retardant properties while maintaining good electrochemical performance in the assembled battery.