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Many studies relating to the preparation of magnetic composites from various raw materials have been reported. However, for the first time, the magnetic Fe‐decorated carbon nanosheets (FeCNs) were prepared using waste facemasks with iron‐containing nose strip as both carbon source and iron source. The uniform decoration of Fe3O4 nanoparticles not only endows carbon matrix with magnetic separation property but also enriches the lamellar structure to furnish more catalytically active sites. The as‐prepared FeCNs nanozymes behave optimal peroxidase‐like activity under pH of 4 and temperature of 50°C, excellent reusability, and substrate affinity. Based on FeCNs, a convenient colorimetric strategy was successfully constructed for flavonoid detection in simulated and real samples. Experimental and theoretical results show that hydroxyl radicals (•OH) and superoxide radical (•O2−) are the major active species in the reactions that are catalyzed by FeCNs in the presence of H2O2. And the generation of these radicals can be attributed to the electron transfer between Fe2+/Fe3+ pairs and H2O2 molecules. This work provides a new way for preparing magnetic carbon‐based nanozymes and realizes the resource utilization of waste facemasks. [ABSTRACT FROM AUTHOR]