A novel phosphorus-nitrogen-silicon polymeric flame retardant (PNSFR) was synthesized via polycondensation and characterized by FTIR, (HNMR)-H-1, and (PNMR)-P-31 The thermogravimetric analysis (TGA) of PNSFR showed that the temperature of 10 wt % weight loss for PNSFR was about 270 degrees C both under nitrogen and air, which indicated that PNSFR exhibited good thermal stability. The crystallization behavior, flame retardancy, dynamic mechanical properties, and thermal stability of polylactic acid (PLA) composites were studied. The results indicated that PNSFR affected the crystallization of PIA and improved the flame retardancy and dynamic mechanical properties of PLA composites. The limiting oxygen index (LOI) of the PLA composites containing 20 wt % PNSFR was about 25.0% compared to 20.0% of pure PLA and attained V-0 rating. After the burning of the PLA composites, compact and coherent charred layer was formed, and the flame-retarded mechanism was discussed in detail.