Abstract: Conventional petroleum-based resin-modified wood suffers from limited functionality, the formation of insoluble and infusible structures after curing, and poor recyclability. To address these issues, a bio-based dynamically cross-linked modified wood was developed. Maleic anhydride-grafted lignin (ML) was first synthesized from maleic anhydride and enzymatically hydrolyzed lignin without the use of solvents. The obtained ML was mixed with furfuryl alcohol (FA) and ammonium dihydrogen phosphate (ADP) to prepare a wood modifier. A composite structure with ADP enrichment in the wood cell walls, reinforced by a dynamically cross-linked resin network, was constructed through a two-step impregnation process combined with densification treatment. By this route, lignin/furfuryl alcohol dynamically cross-linked modified poplar (A-FAML) was successfully fabricated. Flame retardancy was evaluated by limiting oxygen index (LOI), cone calorimetry, and UL-94 vertical burning tests. The results showed that A-FAML exhibited an LOI of 37.7%, a 72.1% improvement over untreated wood. The total heat release and total smoke production decreased by 42.2% and 93.8%, respectively, with a char yield of 41.8%. A-FAML attained a V-0 rating in UL-94 testing while retaining satisfactory flexural properties and leaching resistance. Furthermore, due to the thermoplasticity of lignin and the thermal reversibility of the dynamic cross-linked network, A-FAML also exhibits good recyclability, with the internal bond strength and bending strength of the recycled panels reaching 4.2 MPa and 28.7 MPa, respectively.