With the continuous improvement of living standards, the development of eco-friendly rigid polyurethane foam (RPUF) materials with excellent flame retardancy, thermal insulation, and outstanding mechanical strength has become an urgent challenge. This study presented an innovative strategy using lignin as a mechanical reinforcement and flame-retardant synergist, combined with DMMP and EG as highly efficient flame retardants. Furthermore, the incorporation of a silica aerogel coating via surface post-treatment significantly enhanced the flame retardancy of the composite. Compared to neat RPUF, the Ct-RPUF/L/FR composites exhibited an increased LOI of 25.3 %, a delayed ignition time of 6.0 s, and reductions in total heat release (THR) and total smoke production (TSP) to 8.6 MJ/m