Brominated flame-retardant high-density polyethylene (HDPE) composites containing graphene nanoplatelets (GNPs) were prepared via melt blending. A Lewis acid catalyst, aluminum chloride (AlCl3), was adopted to initiate the Friedel-Crafts reaction, which contributed to the good dispersion of GNPs and improvement of interfacial interaction. Scanning electron microscopy (SEM) showed that the size of GNPs decreased and the dispersive state of GNPs was improved with the addition of AlCl3. The maximum decomposition temperature (T-max), modulus, and viscosity increased as the catalyst content increased, whereas the peak heat release rate (PHRR) and the total heat release (THR) decreased with increasing catalyst content. However, gel permeation chromatography (GPC) verified the degradation for HDPE at high catalyst concentration, and then thermal oxidation stability and flame retardancy worsen. The variety of performance of the composites was the result of the competition between improving the dispersion and degradation of composites.