Abstract: Phytic acid sodium salt (PA-Na), as a green and efficient bio-based phosphorus flame retardant, can coordinate with various metal ions to form an efficient flame-retardant system within wood. Poplar wood was impregnated with PA-Na, metal ions (Al³⁺/Mg²⁺/Ca²⁺), and an alkaline solution. The morphological structure and elemental distribution of the treated wood were characterized by scanning electron microscopy (SEM), infrared spectroscopy (FTIR) and X-ray diffractio (XRD). Flame retardancy was evaluated using limiting oxygen index (LOI) and vertical burning tests (UL-94), while thermal stability and combustion behavior were investigated by thermogravimetric analysis (TGA) and cone calorimetry. The results showed that the LOI of the modified wood increased from 19.6% to 36.5% with rising metal ion concentration, reaching the optimum at 20%, and achieving a V-0 rating in the UL-94 test. TGA revealed that the flame retardant treatment reduced the initial decomposition temperature and accelerated the pyrolysis rate. Cone calorimetry results indicated that the total heat release (THR) of wood treated with PA-Na with 20% Al³⁺, Mg²⁺, and Ca²⁺ decreased by 13.37%, 14.80%, and 10.24%, respectively. Moreover, the smoke release rate (TSR) and CO₂ emission were suppressed during the early combustion stage. Residual char analysis confirmed that the synergistic interaction between PA-Na and metal ions promoted the formation of a protective char layer, which effectively preserved the wood structure and permeability, significantly enhancing the flame retardancy.