The tetrakis hydroxymethyl phosphonium oxalate-urea condensation polymer has phosphonyl group after oxidation and the only three of four primary alcohol groups in THP oxalate react with urea undergoing a crosslinked condensation polymerization. When the THP oxalate-urea polymer is pyrolyzed under atmosphere, phosphorus pentoxide, phosphoric acid or phosphates are formed. On account that phosphorus pentoxide or phosphoric acid formed after being pyrolyzed unnder atmosphere phosphorylates, dehydrates or ${eta}$-eliminates the primary alcohol of C6 of cellulose, cellulose fiber is flame-retarded. The non-oxidated THP oxalate-urea polymer in the P+3 state is thermally very unstabel, and so when the viscose rayon treated with THP oxalate-urea is not oxidated it will not show excellent flame retardancy. The preceding fact was confirmed by DTA curve. This problem phere has conjugated carbon double bond, aldehyde group and furanose ring. Formation of conjugated double bond and decrease of alcohol group seem to result from cellulose being dehydrated or ${eta}$-eliminated by the pyrolysis product of THP oxalate-urea polymer; P2O5H3PO4 or the other phosphates.