Resorcinol bis(diphenyl phosphate) (RDP) is typically used as a flame retardant for the engineering plastics. However, we notice that the RDP has the phosphate groups. The phosphate groups can be considered as the polar moieties. It is reported that the phosphate can act as the good hydrogen bond acceptor. Therefore, the components of intumescent flame retardants (IFR) could form great hydrogen bond with the phosphate groups and the hydrogen bond will reinforce the interfacial adhesion. RDP was used as the compatibilizer of O-SEBS/IFR composites during the melting process. The compatibilization of RDP in O-SEBS/IFR composites was investigated by scanning electron microscopy (SEM) and mechanical properties. SEM micrographs shows the particles were coated with RDP and the addition of RDP led to a decrease in the particles size of IFR. The tensile strength of composites without RDP was 6.46 MPa and increased to 6.87 MPa with the addition of RDP (2 phr), which represented for the better compatibility of O-SEBS/IFR composites. The mechanism of RDP in O-SEBS/IFR composites was proven by temperature-dependent Fourier transform infrared spectroscopy (FTIR) and moving-window two-dimensional correlation spectroscopy (MW2D). The temperature-dependent FTIR spectra clearly show the breaking of two kinds of hydrogen bond. The first one was formed between P=O and –NH4+ of ammonium polyphosphate in IFR, and another hydrogen bond was formed between P=O and –OH of pentaerythritol in IFR. MW2D indicates that there are four processes of the breaking of the hydrogen bond between –NH4+ and P=O and it proves that the P=O of RDP would form hydrogen bond with –NH4+ of ammonium polyphosphate in IFR.