As an alternative to surmount the weakness of natural enzymes, nanozymes have recently attracted extensive attention from early disease diagnosis to tumor therapy. However, due to the lack of enzyme-like molecular recognition units, the poor substrate selectivity of nanozyme-catalyzed reaction significantly restricts their widespread applications. By biomimicking the ingenuity of metabolism process in living, herein we report a multiple-nanozymes system capable of improving the substrate selectivity via cascade reactions in a confined space. As a showcase, the nitrogen-doped carbon nanocages (NCNC) and Prussian blue nanoparticles (PB NPs) were successfully coupled in a microfluidic device to highly selective oxidation and detection of ascorbic acid (AA, an important electron donor and catalyst for alleviation of oxidative stress) against other interferences. It was revealed that the products of the prior nanozymatic reaction were effectively screened as the reactants of the next one, thus stepwise improving the overall selectivity of the all-nanozyme system. This work may open a new door to improve the selectivity of all-nanozyme-system for prospect applications.