Modifies HDPE with nano-magnesium hydroxide(MH)and MH/ RDP-NP composites. Studies the effect of MH's dimension, the amount of MH and silane coupling agents(SC) and the ratio of MH/RDP-NP on the flame retardant property and mechanical property of HDPE. The results show that nano MH is better than micron MH in improving flame retardant property and mechanical property of HDPE. When the mass ratio of HDPE /nano MH is 100/60, the range of vertical burning behaviour can reach FV-2, and the mechanical property decreases about 30 %. By modifying with SC of 3.5 % MH mass, the oxygen index can increase 9.8 % and the impact and tensile strength increase 20 %. When 3:1 ratio of MH/RNP-NP accounting for 40 % HDPE mass, the composites reach the requirement of flame retardancy , the range of vertical burning behaviour is FV-1, its mechanical property decreases less comparing to pure resin, and the decrease range is within 20 %.