Expanded graphite modified with phosphorus (EGP) was synthesized using expandable graphite (EG) and phosphorus oxychloride, and its structure was characterized by infra-red, thermogravimetric (TG), and elemental analysis. Then, EGP was used as a FLame retardant for flexible polyurethane foam (FPUF), and the flame retardant and smoke suppressant properties of the RPUF were investigated by limit oxygen index and cone calorimeter tests (CONE). The thermal degradation behaviors of FPUF samples were studied by TG analysis in air atmosphere. The char structure was studied by scanning electron microscopy. The CONE result indicated that FPUF/EGP showed good flame retardancy and smoke suppression. The TG result showed that for flame retardant FPUF, thermal stability and char residue increased, and that the initial decomposition temperatures decreased. Moreover, the FPUF/EGP showed a lower mass loss than that of FPUF/EG, which indicated that the flame retardancy of EGP for FPUF was improved. The flame retardant mechanism was studied.