In this study, fabrication of manganese-incorporated iron oxide-graphene nanocomposite (rGO-FMBO) was reported for the efficient activation of CaO2 and generation of reactive radicals for the degradation of sulfamethoxazole (SMX). The effects of different systems, catalyst dosage, oxidant dosage, different pH and different reaction time on the degradation of SMX by rGO-APTMS-FMBO/CaO2 as well as the production of free radicals were also studied. Electron paramagnetic resonance (EPR) technique was used to detect and identify the radical species in this oxidation system and these radicals were further confirmed by scavenging studies with the addition of isopropanol (IPA) and methyl viologen (MV2+). The results indicated that the CaO2 could be activated by rGO-APTMS-FMBO efficiently for the effective degradation of SMX at neutral pH (P <= 0.01). The mechanism of the activation of CaO2 by rGO-APTMS-FMBO was that carbon dioxide radicals (CO2 center dot-) generated by rGOAPTMS-FMBO could activate the Ca0 2 to produce more hydroxyl radicals (HO center dot), which favored the SMX degradation. EPR studies showed that three types of free radicals HO center dot, CO2 center dot-, and CH3 center dot were generated and the radical intensities were much higher in rGO-APTMS-FMBO/CaO2 system. Both increased pH and reaction time led to the production of more CO2 center dot-, which activated the CaO2 to give more HO center dot to degrade SMX. Transformation products/intermediates of SMX were determined and potential mechanism and degradation pathway were proposed. The findings of this study provide new insights into the mechanism of heterogeneous catalysis based on CaO2 activated by rGO-APTMS-FMBO and the reactivity of this oxidation system toward environmental contaminants.