Asphyxiation caused by smoke and toxic gas is the main cause of death in fire. How to reduce the release of smoke and toxic gas in the combustion process of polymer materials is very important to protect personal safety and provide rescue in fire. The H·, HO· and other high active free-radicals are released into the gaseous phase produced from the free-radical chain reaction during polymer combustion, which is the fundamental reason of the oxidative degradation of polymer, so it is an effective method to improve the flame retardancy and smoke suppression of polymer through reducing the release of high active free-radicals to the gaseous phase. The free-radical trapping effect in flame retarding and smoke suppression of polymers was reviewed. The different smoke suppression performances of flame retardants/synergists were briefly summarized from two perspectives of the free-radical trapping in gaseous phase and the free-radical trapping in condensed phase. The synergetic effects of the free-radical trapping in condensed phase combined with the covering barrier effect, the catalytic carbonization effect and the porous adsorption effect on delaying and inhibiting the smoke release were introduced emphatically. The problems to be solved on free-radical trapping effect in flame retarding and smoke suppression of polymers were analyzed.