The modified ammonium polyphosphate (MAPP) was obtained by the condensation of hexachlorocyclotriphosphazene (HCCP) and melamine (MEL) onto the surface of ammonium polyphosphate (APP) monomers. Using lignin (Lig) as carbon source, MAPP as acid and gas source to form an expanding flame retardant (IFR) for the preparation of flame-retardant high-density polyethylene (HDPE) cable materials. The results of the limiting oxygen index (LOI) and vertical burning class (UL-94) tests indicate that the modified MAPP/Lig has a higher flame retardant efficiency for HDPE. When the mass fraction of MAPP/Lig is 30% and the ratio is 3/1, the HDPE composite material reaches the vertical combustion V-1 grade, and the LOI value reaches 28.6%. Cone calorimetry (CCT) testing shows that MAPP/Lig flame retardant can significantly reduce the total heat release and heat release rate of HDPE compared to APP/Lig flame retardant. Mechanical test results show that ethylene octene copolymer (POE) can significantly increase the elongation at break of the composite materials. Replacing APP with MAPP will reduce the electrical performance of the HDPE composite materials, but still meet the standard usage requirements.