Abstract: Rigid polyurethane foam (RPUF) exhibits wide applications in building engineering and thermal insulation, as well as furniture components due to its excellent mechanical and insulation properties. However, RPUF is highly flammable, combustion of which can cause serious economic loss and release smoke that is extremely harmful to the environment. Therefore, it’s of great importance to improve the flame retardancy of RPUF. The use of intumescent flame retardants is one of the most economical and efficient ways to protect polymeric materials against fire. In this work, ammonium polyphosphate (APP) and expandable graphite (EG) were added to RPUF as flame retardants. Pyrolysis processes of pure RPUF and these two flame retarded RPUFs were investigated using thermogravimetry (TG), gas chromatography-mass spectrometry (GC/MS) and on-line photoionization mass spectrometry (PIMS). TG analysis shows that all the samples decompose in two major steps and more high-temperature residues are formed with the addition of APP and EG. On the basis of GC/MS analysis, the secondary pyrolysis products of PU are remarkably changed by APP. Large quantities of nitrogen-containing PAHs were found in the pyrolyzates of RPUF/APP, which could subsequently form the char layer to increase the flame retardancy of PU. However, the flame retardancy of EG mainly depends on its physical expansion character. Moreover, extremely similar pyrolyzates are exhibited for pure RPUF and RPUF/EG, which reveals that EG is typically a physical expanding retardant. By virtue of the on-line detection and soft ionization characteristics, PIMS was used to further confirm the above experimental results. Two stages of the pyrolysis process could be assigned respectively to the primary decomposition of RPUF and to secondary reactions of primary products. However, PIMS with rare gas discharge lamp as ionization source has some inevitable drawbacks, such as fixed wavelength, low photon flux and large emittance. Recently, synchrotron vacuum ultraviolet (SVUV) light has proved to be a great choice of ionization source of PIMS. Relative works could be performed with SVUV-PIMS in the future.