Due to efficient performance and stability, nanozymes have recently attracted much attention in bioreaction. In this work, a facile approach for preparing N-doped hollow carbon sheres (N-HCSs) by using CaCO₃ spheres as green template and polydoamine as nitrogen and carbon sources was reported. Morphologies and structures of the samples were characterized. Using TMB (3,3,5,5° tetramethylbenzidine) as a substrate, UV spectra photometry was used to investigate its oxidase-like activity and catalytic mechanism. The results showed that N-HCSs displayed oxidase-like activity. The oxidase-like activity of N-HCSs increased by three times after activation by KOH. These enzymes conform to the Michaelis-Menten kinetic equation, and the K_m constant before and after activation were 0.105 and 0.083, respectively, indicating good affinity for substrates. The data demonstrate that it is superoxide anion (O₂• ^-) that plays a major role in catalytic reaction. This work data provides a theoretical basis for the design and preparation of high activity oxidase-mimicking enzymes.