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A novel flame retardant, tetraethylene pentamine heptamethylene tetraphosphonate dimethyl ester ammonium triphosphorus acid (TPHTDEAT), was synthesized and characterized by NMR. TPHTDEAT-treated cotton fabrics showed excellent flame retardancy tested by the limiting oxygen index (LOI) of 43.3% of cotton fabrics treated with 40 mass% TPHTDEAT and excellent durability tested by LOI of 29.6% of cotton fabrics after 50 laundering cycles according to AATCC 61-2013 2A standard. Most phosphorus existed as phosphonates, which did not combine with metal ions to reduce the flame retardancy during the washing process. P and N atoms were tested by X-ray photoelectron spectroscopy on the treated cotton fibers. The SEM and XRD results implied that flame retardants entered the cotton fiber interiors. The FTIR and durability results suggested that TPHTDEAT was grafted with -OH of cellulose molecules of cotton fibers. The TG analysis suggested that TPHTDEAT promoted char formation during heating process. The cone calorimetry results indicated that the heat release rate and total heat release of TPHTDEAT-treated cotton were much lower than those of untreated cotton. These results indicated that excellent flame retardancy of treated cotton and the durability of treated cotton were increased by introducing phosphonate groups into flame retardant molecules. [ABSTRACT FROM AUTHOR]