Flame-retardant composites for fireproof application of electromagnetic (EM) absorption are produced from the dispersion and formation of graphene nanosheets (GNS) and exfoliated manganese oxides (EMOs) in diphenyl amidophosphate (DAP)-modified epoxy resin. The DAP-modified composite can be a friendly type of high-performance EM absorbing material in practical application, because of its halogen-free merits and high reflection loss in EM absorbance. An overcharring phenomenon is discovered in the retardants with multicomponents embedded in a resin matrix. The flame retardancy of the DAP composite is exhibited from thermogravimetric analysis (TGA). Kinetics analysis presents the difference of composites before and after modification in thermal decomposition. Improvement of multicomponents wrapped system can result from dissociation of P-O bonds during the course of burning. A cone calorimeter is utilized to evaluate the combustion performance to learn about the retardancy of composites. Guarding against overcharring between components, synergistic effect of flame retardancy can be achieved in improving the performance of resin-based absorbers.