In the past decade, nanozymes - a unique class of nanomaterials with inherent enzyme-mimetic properties - have fascinated researchers, revealing unexpected enzyme-like activity of nanomaterials previously considered biologically inert. In particular, as metal-free catalyst for biological processes, carbon-based nanozymes have grown in popularity due to their exceptional physical and chemical characteristics. So far, a variety of carbon-based nanozymes with various structures such as fullerene, graphene oxide, carbon dot, carbon nanotube, and carbon nanosphere have been reported possessing a wide range of enzyme-like properties. However, the structure-activity relationship of the carbon-based nanozymes have not yet been comprehensively discussed. In this review, we thoroughly examine the recent findings on the structure-activity connection of carbon nanozymes, in an effort to comprehend the underlying mechanism of carbon nanozymes and throw light on the future direction of the systematic design and construction of functionally specific carbon nanozymes. We also will address the broad range of applications of carbon nanozymes from in vitro detection to replacing specific enzymes in living systems.