Previously, it was reported that epoxy resin (EP) filled with ammonium polyphosphate (APP) and copper (I) oxide (Cu2O) at a mass ratio of 8/2 (APP8-Cu2O2) as a self-intumescent system demonstrated promising fire retardancy. To further improve the flame retardant efficiency, the possibility of expandable graphite (EG) as an effective synergist for the self-intumescent EP system was revealed by limiting oxygen index (LOI) test. The results showed that the incorporation of EG increased the LOI value of EP/APP8-Cu2O2 obviously. The highest LOI value was obtained at the EG/APP8-Cu2O2 mass ratio of 3/7, indicating the optimal synergistic effect being achieved. Furthermore, UL-94 test results showed that the excellent synergistic effect resulted in the addition of 13 wt% EG/APP8-Cu2O2 of 3/7 not only endowed EP a relatively high LOI value of 34.3%, but also made it pass UL-94 V-0 rating. Moreover, the main fire hazard parameters obtained from cone calorimeter tests, such as peak heat release rate, total smoke production, and peak CO production were reduced 40.3%, 30.3%, and 46.2%, respectively by the combination effect of EG/APP8-Cu2O2 with mass ratio of 3/7, suggesting the excellent improvement in the fire safety of EP significantly. Finally, a possible action mode, which would be beneficial for developing other flame retardant polymers with high fire safety, was proposed.