AbstractHexaphenoxycyclotriphosphazene (HPCTP), modified with phenoxy groups based on hexachlorocyclotriphosphazene (HCCP), has been proved that have excellent thermal stability. In this article, HPCTP and glass fiber (GF) were added into the rigid polyurethane foam (RPUF) to improve the flame‐retardant and mechanical properties of RPUF. The mechanical properties of composite were measured by compressive strength test. The micrographs were observed by scanning electron microscopy. Furthermore, the flame‐retardant and thermal properties were investigated by limiting oxygen index, cone calorimeter test, and thermogravimetric analysis. The results showed that the addition of HPCTP and GF would improve the mechanical and flame‐retardant properties of RPUF.