The development of high-temperature resistant polymers and their composites is of particular interest because of their ever-growing applications. Herein, a family of branched phthalonitrile resins based on different bisphenols (Hydroquinone, 4,4'-Biphenol, m-Dihydroxybenzene, 1,6-Dihydroxynaphthalene, 2,6-Dihydroxynaphthalene) and phenyl-s-triazine, endcapping with 4-nitrophthalonitrile were prepared via a convenient condensation polymerization procedure. The relationship among bisphenol structures, thermal properties and mechanical properties were investigated in details. By incorporating the thermally stable phenyl-s-triazine segment into the branched phthalonitrile's backbone, the overall T-g (glass transition temperature) and T-d (5% thermal degradation temperature) of the derived polymers were significantly improved. The resins were thermally cured to networks with 4,4'-diaminodiphenyl sulfone (DDS) as curing agent. The resulting networks exhibited high thermal resistance, indicated by T-g surpassing 500 degrees C and T-d as high as 575 degrees C. Additionally, the composites exhibited high mechanical properties and dielectric properties stability at 500 degrees C. The results suggested the merits of branched phthalonitrile polymers for applications in high-temperature resistant polymers and composites.