Abstract: Safe and green P-N flame retardants are among the most efficient flame retardants for oxygenated cellulose-based materials. A novel P-N flame retardant, TAM, was synthesized using aminotrimethylene phosphonic acid (ATMP), 1,3,5-tris(2-hydroxyethyl) cyanuric acid (THEIC), and melamine (MEL). The chemical structure of TAM was characterized by FTIR, XPS, and NMR. TAM was then impregnated into fir wood using the full-cell method. The effects of TAM on the thermal degradation and combustion properties of fir wood were analyzed by SEM, TGA, LOI, and CONE tests. The results showed that the flame retardancy of fir wood improved with increasing TAM concentration. When the TAM impregnation solution concentration was 1%, the LOI of 1% TAM/wood reached 34.2%. At a concentration of 5%, the LOI of 5% TAM/wood reached 48.4%. Compared to untreated fir wood (Pristine wood), the flaming combustion time (FCT) of 5% TAM/wood was reduced by 143s. The mean heat release rate (MHRR), total heat release (THR), and total smoke production (TSP) decreased by 56.8%, 70.6%, and 73.6%, respectively, while the residual char yield increased from 19.46% to 43.58%. Thermogravimetric analysis indicated that the maximum thermal degradation rate temperature of the wood shifted from 369.8℃ for Pristine wood to 296.3℃ for TAM-treated wood.The study concludes that TAM effectively enhances the flame retardancy of fir wood through a synergistic mechanism involving both gas-phase and condensed-phase actions. The decomposition of TAM produces non-combustible gases, and the phosphorus-containing compounds catalyze the charring of fir wood to form a continuous expanded char layer, which hinders the transfer of mass and heat and retards the degradation rate of the wood. This work provides a new strategy for the development of efficient and environmentally friendly P-N flame retardants.