Nanomaterials doped with non-metallic C have attracted tremendous attention as potential nano-artificial enzymes due to their ability to change the energy band structure to improve their intrinsic properties. Herein, we report a green, facile, efficient, and fast strategy to access high-performance nanozymes via supercritical CO₂ fluid technology-fabricated polymer nanoreactor of poly-(methyl vinyl ether-co-maleic anhydride) (PVM/MA) coated Co(NO₃)₂ into C-doped Co₃O₄ (C-Co₃O₄) nanozyme by a one-step calcination process. Converting PVM/MA to C doping into Co₃O₄ shortens the entire lattice constant of the crystal structure, and the overall valence band energy level below the Fermi level shifts toward the lower energy direction. The as-prepared C-Co₃O₄ demonstrated significant peroxidase-like catalytic activity, significantly greater than the undoped Co₃O₄ nanoparticle nanozyme. The following density functional theory (DFT) calculations revealed that the doped nano-enzyme catalytic site displayed a unique electronic structure, altering the material surface with more electrons to fill the anti-bond of the two molecular orbitals, and significantly improving the peroxidase-like enzyme catalytic and glucose sensor performance. The resultant enzymatic glucose sensing in a linear range of 0.1–0.6 mM with a detection limit of 3.86 μM is in line with standard Michaelis–Menten theory. Collectively, this work demonstrates that converting polymers into nanozymes of C-doped form by supercritical CO₂ fluid technology in a step is an effective strategy for constructing high-performance glucose sensor nanozymes. This cost-effective, reliable, and precise system offers the potential for rapid analyte detection, facilitating its application in a variety of fields.