In summary, a new nanocomposition-based peroxidase mimic with typical core–shell structure has been developed and its synergetic peroxidase-like activity has for the first time been demonstrated. It has been found that the presence of carbon shell contributes to the improvement of water-dispersion and peroxidase-like activity. Owing to the synergetic interactions between magnetic core and carbon shell, an enhanced peroxidase-like activity can be observed from Fe3O4@C as compared with its two components. As a peroxidase mimic, Fe3O4@C follows Michaelis–Menten kinetic behaviors and exhibited higher affinity for H2O2 and TMB than HRP. The oxidation mechanism of TMB catalyzed by Fe3O4@C can be attributed to the generation of radical dotOH from the decomposition of H2O2. By combining the catalytic reactions of Fe3O4@C and GOx, a simple colorimetric method for glucose detection has been developed. The proposed method exhibited high sensitivity and selectivity. Furthermore, the levels of glucose in serum and urine samples can be measured accurately by using the Fe3O4@C-based colorimetric method.