Abstract: The effect of Mg(OH)2 on combustion characteristic of polyvinyl chloride (PVC) was investigated by means of cone calorimeter,smoke density instrument and electrical conductivity test.The HCl releasing rule in the system and the compounded method that could show the synergistic effect and furthest decrease the formation amount of HCl were discussed.The results show that after adding the Mg(OH)2 into the PVC,the initial decomposition temperature of the PVC increased slightly,compared with the pure PVC the carbon left amount of the system obviously increased,that improved the PVC pyrolysis and the carbon bone frame formation.The absorption of smoke vs.light and mass loss rate of the Mg(OH)2 reached the maximum within 300-500 s,and then decreased after 500 s.The maximum synergistic effect could be obtained after mixed the Mg(OH)2 with Sb2O3,the maximum and the average absorption of smoke vs.light as well as the pyrolysis residue were 6.38%,10.34% and 1.32 times of PVC with single addricon of Sb2O3.Mg(OH)2 could decrease the HCl releasing amount of the system.