Flame‐retarded rigid polyurethane foam (RPUF) conferred by an amino trimethylphosphonate iron (Fe2+‐ATMP) and expandable graphite (EG) compound flame retardant has been successfully fabricated by one‐step all‐water foaming method. The combustion characteristics of the RPUFs and the synergistic effect of Fe2+‐ATMP and EG on RPUF have been studied based on limiting oxygen index (LOI), smoke toxicity analysis, cone calorimeter (CONE), thermogravimetric (TG) analysis and thermal decomposition kinetics analysis. The current results indicated that the flame retardancy and thermal stability of the RPUFs modified by Fe2+‐ATMP and EG had been improved obviously and the contents of smoke toxic gases had been significantly reduced. When the ratio of Fe2+‐ATMP to EG was 1:5, LOI of the modified RPUF was the highest (27.1%). And its peak heat release rate (PHRR) and total heat release (THR) were the lowest, which were 135 kW/m2 and 15.7 MJ/m2, respectively, and were 40.5% and 17.8% lower than RPUF‐0. It also showed that the modified RPUF with a Fe2+‐ATMP to EG ratio of 1:5 had the best flame retardancy, smoke suppression, and thermal stability. The present work can provide some reference and guidance for flame retardant modification of polyurethane foam.