Abstract: To address the high heat release and smoke generation of epoxy resin (EP) during combustion, as well as the difficulty in simultaneously enhancing its flame-retardant and mechanical properties, a novel DOPS-based nitrogen-containing heterocyclic flame retardant, TGED, was synthesized through molecular structure design. In this molecule, bio-based eugenol structural units, triazine-trione rings, and a phosphorophilli-monomer 9,10-dihydro 9-oxa-10-phosphorophilli-10-sulfide (DOPS) groups were integrated into a single flame-retardant molecular framework. The synthesized TGED was subsequently applied to the flame-retardant modification of EP. The chemical structure of TGED was confirmed by FTIR, NMR, and HR-MS. The comprehensive properties and flame-retardant mechanism of the modified EP were systematically investigated using TG, LOI, UL-94, cone calorimetry, SEM, Raman spectroscopy, and TG-FTIR. TGED altered the thermal degradation behavior of EP, reduced its mass-loss rate, and promoted char formation at elevated temperatures. At a loading of 10wt%, TGED-10/EP exhibited a char yield of 34.02% at 700℃, an LOI of 33.8%, and a UL-94 V-0 rating. Compared with neat EP, its peak heat release rate, total heat release, and total smoke production decreased by 61.98%, 65.64%, and 61.91%, respectively, while its flexural, tensile, and notched impact strengths increased by 23.49%, 30.64%, and 37.15%, respectively. Thus, TGED simultaneously enhanced the flame retardancy and mechanical properties of EP, alleviating the conventional trade-off between these properties. Mechanistic analysis demonstrated that TGED exerted its flame-retardant effect through the combined actions of gas-phase flame inhibition and condensed-phase char protection. This study provides a new molecular design strategy for developing high-performance flame-retardant EP.