Pentaerythritol phosphate melamine salt (PPMS) was synthesized from pentaerythritol, phosphoric acid and melamine, and applied to flame retardant polyethylene (PE). It was characterized by infrared, thermogravimetric analysis, vertical combustion, limiting oxygen index (LOI), cone calorimetry and mechanical properties. The results showed that the optimum process for PPMS synthesis was as follows: 110% polyphosphoric acid reacted with pentaerythritol at 110 °C for 5 h, and the synthesized pentaerythritol phosphate reacted with melamine at 90 °C for 5 h. The 1% and 5% thermal decomposition temperature of the product were 200.37 °C and 229.85 °C, respectively. Compared with PE samples, the LOI of the composites increased significantly from 17.6% to more than 24%, and the peak heat release rate, total heat release, average effective heat of combustion and total smoke production decreased significantly. The charring performance and flame retardancy of the composites were significantly improved, while the mechanical properties decreased. When the amount of flame retardant was 30% ( including charring agent ) and 35%, the flame retardancy of the composites were more significant and basically the same.