The excellent performance of rigid polyurethane foam (RPUF) makes it widely used in various fields, but its flammability has laid an enormous security risk in the application process. As a solid waste discharged after refining iron concentrate, steel slag (SS) causes environmental pollution problems due to its low comprehensive utilization efficiency. To improve the fire safety of RPUF and the added value of steel slag, SS and aluminium hypophosphate (AHP) were introduced into RPUF. A series of flame retardant RPUF (FR-RPUF) composites were prepared using one-step water-blown method. The chemical composition of SS was investigated by X-ray fluorescence spectroscopy (XRF), and the effects of SS/AHP on the microstructure, thermal stability, flame retardancy and gas phase products of FR-RPUF composites were systematically studied. XRF test showed that the chemical components of steel slag were mainly CaO, SiO2, Al2O3, Fe2O3, SO3, MgO, et al, which could be used as synergistic agents to promote the char formation of polymer and cover the surface of the material to block combustion heat, to achieve the purpose of condensed phase flame retardant. Scanning electron microscope (SEM) test indicated that the poor compatibility between SS/AHP and RPUF matrix led to varying degrees of damage of FR-RPUF cells. The flame retardant tests made known that the limiting oxygen indexes of RPUF-1, RPUF-4, and RPUF-5 were 24. 2 vol%, 23. 4 vol%, and 19. 7 vol%, respectively, indicating that SS and AHP had a synergistic flame retardant effect. The RPUF/SS/AHP composites all passed UL-94 V-0 level, meeting the requirements of external wall insulation materials. Thermogravimetry-infrared (TG-FTIR) analysis showed that SS/AHP did not change the degradation process of RPUF. The gaseous products were mainly hydrocarbons, carbonyl compounds, carbamates, CO2, isocyanate compounds, aromatic compounds, hydrogen cyanide, carbonyl compounds and esters. TG-FTIR tests implied that RPUF and FR-RPUF generated carbamates, hydrocarbons, CO2, isocyanates, carbonyl compounds, aromatic compounds, esters and hydrogen cyanide. Further analysis of typical volatile products showed that the addition of AHP and SS promoted the early degradation of RPUF matrix. Raman test confirmed that the peak area ratio (ID/IG) of D peak to G peak of RPUF-4 was the smallest when SS/AHP was added to RPUF, indicating that the addition of SS/AHP improved the compactness and graphitization degree of FR-RPUF composites. The flame retardant mechanism was proposed. Firstly, AHP endothermic decomposition reduced the temperature around the flame, and the decomposed PO free radicals captured free HO ·to inhibited the chain reaction. Secondly, the decomposed aluminum pyrophosphate and char layer were covered on the substrate surface, which inhibited the diffusion of flame and combustible gas. The PH3 produced by AHP decomposition reacted with oxygen to form acidic substances (phosphoric acid and polyphosphoric acid), which promoted the dehydration of RPUF into char. The mineral components in steel slag reacted with phosphoric acid or polyphosphoric acid to form a dense char layer, which played a synergistic flame retardant effect with AHP.