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A novel flame retardant named diethylene N,N',N''‐tri (diethoxy)phosphoramide (DTP) was synthesized using diethyl phosphate and diethylenetriamine via Atherton–Todd reaction. Then, series of flame‐retardant water‐blown rigid polyurethane foams (RPUFs) with expandable graphite (EG) and DTP were prepared through box‐foaming. The results of thermogravimetric analysis showed that DTP/EG changed thermal degradation process of RPUF and promoted enhancing char residues. The complex flame‐retardant system (EG/DTP) endowed RPUF higher limiting oxygen index (LOI) values (29.1%–30.2%) and lower heat release rate peak (PHRR) values according to LOI and microscale combustion calorimeter tests. More importantly, the synergistic flame‐retardant effect between EG and DTP in RPUF was proved by the analysis of synergistic effectivity values. Based on the analysis of cone calorimetric tests, EG/DTP revealed remarkable effects to inhibit the fire intensity and smoke release of RPUF with decreased PHRR and total smoke production due to good char‐forming action. To further investigate the char‐residues of the foams after combustion, scanning electron microscope and energy dispersive X‐ray spectroscopy analyses were conducted. The results suggested that EG/DTP flame‐retardant system promoted RPUF forming a compact, continuous and phosphorus‐rich char layer as a good fire barrier in combustion. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46434.

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