In order to prepare a synergetic flame retardant, the magnesium hydroxide (MH) and hexaphenoxycyclotriphosphazene (HPCTP) were blended with ethylene-vinyl acetate copolymer (EVA). The EVA-MH-HPCTP composite materials were characterized by various tests including Fourier transform infrared spectroscopy, X-ray diffractometer and scanning electron microscopy (SEM). The SEM images indicated that the compatibility of EVA-MH-HPCTP composite materials was significantly promoted due to the bridging effect of HPCTP. Therefore, the EVA-MH-HPCTP composite materials possessed better processability which was evidenced by the melt flow rate tests. Comparing to the traditional addition dosage (over 50%) of MH in previous reports, the EVA-MH-HPCTP composite materials showed significant flame-retardant efficiency even when the dosage of MH was lowered to 20 parts per hundred resins. The experimental results suggest that the prepared EVA-MH-HPCTP composite material may be a potential candidate in various industrial applications due to its excellent flame retardant.