Abstract: The fire retardant and smoke suppression performance of thermal insulation materials are crucial factors in ensuring building fire safety. In this study, based on ammonium polyphosphate (APP), a intumescent flame-retardant APP@AL (Mass ratio APP∶MDI∶AL=9∶2∶1) with core-shell structure was prepared by utilizing alkali lignin (AL) and bis(4-isocyanatophenyl) methane (MDI) for its poor interfacial compatibility and smoke suppression performance, and applied to the flame-retardant treatment of rigid polyurethane (RPU) thermal insulation materials (25wt% addition). The structure and composition of APP@AL were analyzed by SEM-EDS, XPS and FTIR, and the flame retardant and smoke suppression properties were analyzed by CONE and TG instruments. The results show that APP@AL has excellent interfacial compatibility with the RPU matrix. The compressive strength of the APP@AL-modified RPU foam is significantly increased (up to 31.8%) compared with the APP-added RPU foam and the thermal conductivity is reduced (7.0%). CONE test show that mean heat release rate and total heat release are reduced by 27.2% and 24.4%, respectively, while smoke suppression is notably enhanced (47.6% reduction in total smoke production and 57.0% reduction in total CO production). TG analysis show that the thermal stability of APP@AL flame retardant is obviously higher than that of APP, and it is more beneficial to build a stable residual char layer. In conclusion, lignin-based intumescent flame retardant has excellent flame retardant and smoke suppression effect on RPU foam insulation materials.