A novel phosphorous/benzene-containing flame-retardant diphenyl phosphoryl polyethylene (PEP) is synthesized from ethylene vinyl acetate (EVA) copolymer with two steps reactions and its chemical structure is characterized by Fourier transform infrared and nuclear magnetic resonance spectroscopies. The grafted molecular dispersion is calculated by gel permeation chromatography and the phase transition is analyzed by differential scanning calorimetry. And results show its promising processability potential. High density polyethylene (HDPE) composite containing 15 wt% PEP is prepared by melt blending, and its fire performance and thermal behaviors are investigated in terms of limiting oxygen index (LOI), vertical burning (UL-94) and thermogravimetric analysis. There is a 16% increase to 20.2 in the LOI value of PEP, while it fails the UL-94 test. The char morphology is characterized by scanning electronic microscopy and observed multi-layered intumescent residue char effectively reduces the transfer of heat and oxygen. The major degradation is quantitively analyzed by the Flynn-Wall-Ozawa method. The results demonstrate that there is a catalytic effect by the phosphorous substance in char formation. Finally, the mechanical properties tests show that the loss due to the addition is small, which signifies the good compatibility between the pure HDPE and the grafted addition.