This work aims to research rigid polyurethane foam (RPUF) and rigid polyurethane foam/expanded graphite composites (RPUF/EG). Thermogravimetric analysis-Fourier transform infrared spectrophotometer (TG-FTIR) was applied to investigate generation and change law of gaseous phase products in combustion process. Scanning electron microscope (SEM) and X-ray photoelectron spectrometer (XPS) were used to investigate micro-morphology, elemental composition and bonding state of the char residue, which combined to flame retardant tests to discover flame retardant mechanism of PRUF/EG composites. SEM analysis revealed the existence of a lot of wormlike structure in char residue of RPUF/EG composites. TG-FTIR revealed that RPUF/EG composites presented two stages in decomposition process; the first one corresponded to decomposition of hard segment of polyurethane molecular chain, while the seconded one corresponded to decomposition of soft segment of polyurethane molecular chain. The pyrolysis products of RPUF and RPUF/EG composites included isocyanate compound, amine compound, hydrocarbons, aromatic compounds, CO, CO2 and esters, and RPUF/EG presented higher intensity of decomposition products for hard segment compared with RPUF. XPS revealed that char residue of RPUF contained C of 77.63%, N of 10.30%, O of 12.07%, while char residue of RPUF/EG30 contained C of 82.18%, N of 9.18%, O of 8.35%. Furthermore, peak fitting for bonding state of C element showed that C—C/C—H, C—O/C—N and CO/CN structure in char residue of RPUF were 51.38%, 38.89% and 9.73%, while those in char residue of RPUF/EG30 were 53.99%, 37.62% and 8.39%, indicating EG promote C element in polyurethane molecular chain to form stable graphite carbon structure, which was beneficial to form compact char layer. Peak fitting for bonding state of N element showed that —NH— and N structure in char residue of RPUF were 49.06% and 50.94%, while those in char residue of RPUF/EG30 were 43.96% and 56.04%, indicating EG promote N element in polyurethane molecular chain to form stable aromatic heterocyclic structure, which was also beneficial to form compact char layer. Peak fitting for binding state of O element showed that O, —O— and O2/H2O structure in char residue of RPUF were 19.30%, 16.72% and 63.98%, while those in char residue of PRUF/EG30 were 25.57%, 36.60% and 37.83%, indicating that the significant enhancement of compactness for the char residue of RPUF/EG composites. Based on TG-FTIR, XPS, SEM and flame retardant tests, flame retardant mechanism can be obtained as follows: EG particles expanded to form wormlike structure and released acid gas, which promoted the degradation of hard segment of polyurethane molecular chain. At the same time, EG promoted C and N element in polyurethane molecular chain to form compact char layer, which combined with wormlike structure to cover on the surface of combustion area, significantly inhibited mass and heat transmission, thus achieving the purpose of flame retardancy. The above researches provided experimental and theoretical basis for discovering flame retardant mechanism of EG and expanding further application in relative fields.