This paper focuses on revealing the thermal decomposition behaviors of a self-intumescent flame retardant (IFR) epoxy (EP) resin (EP/15%APP-Cu2O) employing 12 wt% ammonium polyphosphate (APP) as a halogen-free flame retardant and 3 wt% copper (I) oxide (Cu2O) as char forming rate regulator. Initially, the thermal stability of EP/15%APP-Cu2O was analyzed and compared to virgin EP resin and flame retardant EP (EP/15%APP) containing 15 wt% APP as flame retardant by thermogravimetric analysis test at different heating rates under nitrogen atmosphere. It was shown that the incorporation of APP altered the decomposition pathway of EP and decreased the onset decomposition temperature. Luckily, compared to EP/15%APP, the onset decomposition temperature of EP/15%APP-Cu2O was just slightly reduced from 300.4 to 292.8 degrees C. Then, the thermal degradation kinetics of EP, EP/15%APP and EP/15%APP-Cu2O were further evaluated by Kissinger and Flynn-Wall-Ozawa methods. It was worth noted that the addition of APP or APP-Cu2O enhanced the thermal degradation activation energies of EP, which contributed to the protective effect of the char formation. Particularly, the incorporation of 3 wt% Cu2O significantly decreased the thermal degradation activation energies at the early decomposition stage of EP. This may be the main contribution for intumescent char formation, which resulted in higher fire safety of EP/15%APP-Cu2O compared to EP/15%APP. These information can potentially help to develop alternative IFR systems.