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In this work, a series of phthalonitrile/hollow glass microsphere (HGM) composite foams for applications in the high temperature and flame retardant fields were prepared and characterized. First, Blends of bisphthalonitrile (CN) and alicyclic imide compounds synthesized from tetrahydrophthalic anhydride (CC) were prepared and sieved (>100 mesh), based on the Thermal Synergistic Polymerization (TSP) effect between phthalonitrile and the alicyclic imide compounds, namely, the -C≡N of the phthalonitrile compound could be completely and rapidly consumed. Then thermogravimetric analysis and rheological behaviors of different molar ratios of CC/CN blend at different temperatures were investigated. HGM were modified using a silane coupling agent (KH550) to improve the interface interaction between HGM and the resin, thereby improving the properties of composite foam. Finally, a series of composite foams were fabricated and characterized. The performance of the foam prepared by BMC method is much better than that of the foam prepared by “RTM” method. The foam (BMC) shows a density of 0.47 g/cm3 with no obvious volume change and little voids, also exhibits good thermal stability up to 400 °C and low thermal conductivity, and provides UL94 V0-rated flame retardancy with total HR of 21.5 KJ/g.