The investigations on the flame retardants and their flame retardant mechanism have be drawn increasing attentions recently.The flame retardant mechanisms of halogenate flame retardant, phosphorous flame retardant,nitrogen flame retardant,Al(OH)_3 and Mg(OH)_2 flame retardant,clay flame retardant and expandable graphite flame retardant used in polymers and their current study are reviewed in detail.Finally,a novel idea for the design of flame retardant,that is to say,to increase the charring tendency during the burning of composite materials,is put forward.