A novel additive, tri(phosphaphenanthrene-maleimide-phenoxyl)-triazine (DOPO-TMT), was successfully synthesized. The chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance. DOPO-TMT was blended with epoxy resins to prepare flame-retardant thermosets. The flame-retardant properties were evaluated using limited oxygen index (LOT), vertical burning (UL94), and cone calorimeter tests. The results indicated that DOPO-TMT exhibited excellent flame-retardant effect. The flame-retardant mechanism was studied by thermogravimeric analysis (TGA), pyrolysis-gas chromatography/mass spectrometry, and thermogravimetric analysis/infrared spectrometry (TGA-FTIR) coupled with the morphology and chemical analysis of the char residues. The results disclosed that DOPO-TMT exerted biphase flame-retardant effect. In gaseous-phase, DOPO-TMT released phosphorus- and nitrogen-containing free radicals with quenching effect under thermal decomposition. The morphologies of the char residues exhibited intumescent and honeycombed structure with a small number of holes on the surfaces. The honeycombed char structure served as an excellent protective layer. A few number of holes on the surface facilitated the concentrated release of free radicals to implement a strong quenching effect. The functional groups of DOPO-TMT synergistically interacted to endow epoxy resin with excellent flame retardancy.