Polymer foams are extremely flammable due to their composition (mainly carbon and hydrogen) and high porosity, thin pore wall, large surface area, and high content (~98%) of air, which makes it much more difficult to flame retard the foams than the bulk polymers. Therefore, halogen-free flame retardant polymer foams are urgently needed, but also remain a big challenge worldwide. In this paper, two novel halogen-free flame retardant polymer foams, i.e., expendable polystyrene (EPS) foam, the largest quantity of thermal insulation material used in building energy saving, as well as poly (vinyl alcohol) (PVA) foam, with intrinsic flame retardant characteristics, were reported. Novel halogen-free flame retardant EPS foam with good comprehensive performances was successfully prepared by strengthening the condensed phase retardancy and promoting the char forming and improving the char layer quality. The prepared EPS foam could achieve UL-94 V0 rating test, 40% of LOI, and 11.2 of smoke density, also possessed good mechanical and processing properties, e.g. good flexural strength and compression strength, low water absorption and thermal conductivity coefficient. Novel halogen-free flame retardant PVA foam with intrinsic flame retardant characteristics was successfully prepared based on our invented thermal foaming technology of PVA using water as blowing agent, in which PVA acted as the charring agent, stabilized by interaction with N-P containing flame retardant, and the bound water in PVA foam acted as the synergistic flame retardant. The prepared halogen-free flame retardant PVA foam could achieve UL94 V-0 rating, 35% of LOI, and combined with pretty good mechanical properties, which is a big breakthrough in the field of halogen-free flame retardant polymer foams. Keywords: Halogen-free flame retardant polymer foams, Expendable polystyrene (EPS) foam, Poly (vinyl alcohol) foam.