Exploring renewable bio-based polymer materials with intrinsic flame retardancy and high mechanical strength will greatly increase the potential to meet fire safety and practicability requirements. In this study, a hexasubstituted cyclophosphazene monomer (HEP) is synthesized from abundant and renewable eugenol and then copolymerized with multi-thiol monomers to establish HEP-SH polymer networks by thiol-ene photopolymerization. Four HEP-SH polymer networks (HEP-TEGDT, HEP-TTMP, HEP-PETMP, HEP-TEMPIC) were prepared by varying the number of thiol groups or the backbone of thiol monomers. Among these polymer networks, HEP-PETMP and HEP-TEMPIC networks exhibit excellent flame retardancy (LOI > 27 vol.%, achieved UL 94 V-0 rating). The combustion parameters and flame retardant mechanism are further studied by cone calorimetry and combined scanning electron microscopy/energy-dispersive spectroscopy (SEM-EDS), which confirm the intumescent flame retardancy of HEP-SH networks is simultaneously affected by the contents of phosphorus and nitrogen element, as well as the structural features of the polymer network. In addition, the HEP-TEMPIC and HEP-PETMP networks also exhibit high mechanical strength and transparency. These eugenolbased thiol-ene polymer networks open a new pathway to develop sustainable high-performance flame retardant bio-based polymer materials for practical application.