In this work, Cu2O exhibited better synergistic effect with microencapsulated ammonium polyphosphate (MAPP) than other metal oxides, such as CuO, ZnO, SnO, Fe2O3, Ni2O3, and Co2O3, on improving the flame retardancy of epoxy resins (EP). Moreover, the addition of cuprous oxide (Cu2O) could further decrease the total smoke production and carbon monoxide production. The analysis of char residues and evolved gases indicated that Cu2O was beneficial to enhance the formation amount, intumescent degree, and compactness of char. The protective layer of intumescent char residue could hinder the decomposition of EP and diffusion of gaseous products, such as hydrocarbons, aromatic compounds and carbon monoxide (CO). In addition, Cu2O also played a role in conversion of CO to CO2 through a redox cycle, involving the reduction of Cu2+-Cu+-Cu-0 by CO and the oxidation of Cu-0-Cu+-Cu2+ by O-2. The possible flame retardant, smoke suppression, and CO oxidation mechanisms were proposed.