A flame retardant synergist (g-C₃N₄@Fe₃O₄) was designed through an in situ co-precipitation method by using graphitized carbon nitride (g-C₃N₄) and Fe₃O₄, and its structure was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). A series of different contents of g-C₃N₄@Fe₃O₄ and intumescent flame retardant (IFR) were simultaneously added into eucommia ulmoides gum/natural rubber (EUG/NR) blends to improve the flame retardancy and reduce the smoke release. The flame retardancy, smoke suppression performance, thermal degradation behaviors and thermal stability of EUG/NR blends were evaluated by the limiting oxygen index (LOI), UL-94, cone calorimetry test (CCT) and thermogravimetric analysis (TGA). The results showed that using g-C₃N₄@Fe₃O₄ as a flame retardant synergist, the LOI value of the 9 phr g-C₃N₄@Fe₃O₄ synergist (EUG/NR 5) blend was 29.5%, which was much higher than the 20.0% of the original composite. Moreover, the 6 phr g-C₃N₄@Fe₃O₄ synergist (EUG/NR 4) and EUG/NR 5 passed the UL-94 V-0 rating, while the pristine EUG/NR blends showed no rating. Moreover, the total heat release rate (THR), peak of heat release rate (PHRR) and total smoke production (TSP) of the EUG/NR blends in the CCT test were much lower than those of the pristine EUG/NR blends. In particular, PHRR, THR and TSP of EUG/NR 5 decreased by 37.1%, 16.2% and 18.0%, respectively. This indicated that the introduction of g-C₃N₄@Fe₃O₄ decreased the release of combustible gases. TGA results also showed that the addition of g-C₃N₄@Fe₃O₄ accelerated the thermal degradation of the EUG/NR blends and changed the thermal degradation mechanism of the EUG/NR blends, indicating the synergistic effect of g-C₃N₄@Fe₃O₄. Finally, a possible degradation mechanism of EUG/NR blend composites was proposed.