Abstract: Many metal oxide nanozymes have been demonstrated to exhibit a peroxidase-like activity much higher than an oxidase-like one in triggering the oxidation of colorless TMB to blue oxTMB. Unlike the conventional phenomenon, here, we observe that amorphous CoO_x can show strong oxidase-like activity, but its activity decreases after the addition of H₂O₂. Further experimental investigation revealed the underlying mechanism: H₂O₂ decomposes CoO_x into inactive cobalt ions. Through redox potential calculations, the feasibility of their reaction was theoretically confirmed. Moreover, the unusual phenomenon mainly occurs in amorphous CoO_x and does not exist in cobalt oxides with distinct crystallinity. Based on the finding, a biocatalytic enzyme–nanozyme cascade system can be constructed using CoO_x and glucose oxidase (GOx) for detecting glucose in human serum. In the presence of oxygen, glucose is catalyzed by GOx to generate H₂O₂, which decomposes the CoO_x nanozyme, reducing its oxidase-like activity and thereby inhibiting the TMB coloration system. Therefore, with the increase in glucose concentration, the absorbance of the color-developing system also gradually decreases, thus achieving the quantitative detection of glucose. The developed method has high sensitivity and also shows good anti-interference ability in real samples, providing a new strategy for blood glucose detection based on nanozymes.