Abstract: Ammonium polyphosphate (APP), MoO3, expandable graphite (EG), and micro-capsulated red phosphorus (MRP) were used as the synergistic flame retardants of the PE/Ca(OH)2 composite [Ca(OH)2 content 35wt%]. The mechanical property and flame retardancy of the composite were studied and the thermal behavior of the composite was analyzed by TG in nitrogen circumstance. It showed that the oxygen index of PE/Ca(OH)2 composites containing a few synergistic agent（APP，MoO3，EG，MRP）were improved by 2.38%, 4.76%, 7.14%, and 26.19%, respectively, compared with the default system. More solid and steady charcoal layer would be built during the process of thermal oxidation degradation.

Key words: ammonium polyphosphate, molybdenum trioxide, expandable graphite, high-density polyethylene, calcium hydroxide, synergistic flame retardant, micro-capsulated red phosphorus