<p>Densified wood (DW) is a promising sustainable structural material with excellent mechanical performance, whose fire safety can be significantly enhanced through flame-retardant treatment. However, comprehensive pyrolysis models for flame-retardant densified wood (FRDW) remain limited. In this study, ammonium dihydrogen phosphate (ADP)-impregnated densified wood (DW-ADP) was prepared and its pyrolysis and combustion behaviors were investigated through experiments and numerical simulations. Compared with untreated DW, DW-ADP exhibited lower pyrolysis temperatures, increased char yield, delayed ignition, and reduced heat release due to the catalytic effect of ADP. Pyrolysis models for DW, ADP, and DW-ADP were developed via inverse analysis and parameter optimization using microscale characterization data. Initial DW-ADP models based on either parallel integration of DW and ADP reactions scheme or full seven-step consecutive reaction scheme reproduced microscale characterization curves but failed to predict combustion characteristics. A modified ‘1 + 6’ reaction scheme, incorporating an extractive pyrolysis reaction in parallel with six consecutive reactions, was therefore proposed. This model accurately reproduced experimental combustion behavior, with prediction uncertainties below 20% for key parameters under 50 kW m^− 2 irradiation. These results provide a robust basis for predictive fire modeling and support the rational design and broader application of FRDW in fire-safe structural materials.</p>