Flexible polyurethane (PU) foam is one of the most versatile materials, which has a widely range of applications. However, flexible PU foam it is easily ignitable and highly flammable. This study developed a kind of post-modification method to prepare inherent FR PU foam, which use beta-CD as chain extender and introduce FR additive through a facile esterification between phosphoric acid and beta-CD moiety in PU foams. The morphology and elasticity of PU still maintained after the modification of PU. beta-CDs, as a bioresources, can not only act as polyol carbon sources to promote carbon formation, but also crosslinked with phosphate ester and PU matrix through P-O-C bonds to form compact carbon layer during the combustion. This dense carbon layer effectively isolates the heat source of the flame, resulting in great improvement of combustion resistance. The time to ignition for 15% CD-H3PO4-PUF foam reached 152 s, which was over 10 times longer than that of pure PU foams. And the foam immediately self-extinguished when the flame was removed. The THR of 15% CD-H3PO4-PUF foam decreased by 70.43% compared to that of neat PU foam. The significant delay of TTI and reduction of THR can reduce the hazards of flame spread greatly. (C) 2020 Elsevier Ltd. All rights reserved.