Abstract: Halogen-free synergistic flame retardant system was composed of ammonium polyphosphate, pentaerythritol and molybdenum trioxide. The influences of the amount of MoO3 on properties of flame retardant system composite phendic foans (FRSCPFS) including heat release, smoke release and mechanical properties, etc, were investigated. The results showed that compared with pure phenolic foam, average limit oxygen index (ILO) of FRSCPFs increased around 73%. The residual carbon (900℃) increased slightly when MoO3 amount≤1.5%, and then decreased. Heat release rate (HRR), total heat release (THR), oxygen consumption (O2C), and production of carbon monoxide and carbon dioxide (COP and CO2P) reduced about 75%, 68%, 68%, 28% and 41%, respectively. Average total smoke release (TSR) increased about 72%. Average rate of heat emission (ARHE) decreased slightly when MoO3 amount≤1.5% and then increased. The flame retardant system was proved that flame retardant mainly played a role in the gas phase and agreed with the gas phase flame retarding mechanism by the increase of specific extinction area (SEA) and by the effect heat of combustion (EHC). Average bending and compression strength reduced about 29% and 19%, respectively. By comprehensively comparing various performances, the properties of FRSCPFs were best when the amount of MoO3 was 1.5%.