The work aims to summarize the strengthening mechanism and design criteria of PVD nano-coating from the perspectives of nano-multilayer coating and nano-composite coating, to propose the two main issues as poor applicability of coating strengthening mechanism and lack of experimental means to prove theoretical problems existing in the current coating strengthening mechanism and coating design criteria and analyze the causes of the problems reasonably. The research status of the hardening mechanism of PVD nano-coating at home and abroad was reviewed from the perspectives of nano-multilayer coating and nano-composite coating. The research revealed that the strengthening mechanism and some coating characterization experiments played significant roles in the study of coating hardening mechanism. The researchers analyzed the hardening mechanism of the coating according to the strengthening mechanism and necessary test methods. However, the strengthening mechanism of coating was not an essential tool for the study of the hardening mechanism of coating. There was not only one or more coating strengthening mechanisms to illustrate the coating hardening mechanism, but also no coating strengthening mechanism to explain the coating hardening mechanism. In view of this phenomenon, the internal and external factors were analyzed. The external factor was that the combination of coating materials and experimental variables were different; and the internal factor was that the coating strengthening mechanism and design criteria were not perfect. Finally, combined with the existing issues in the research on the hardening mechanism of PVD nano-coating, the future research direction was proposed based on the purpose of improving coating strengthening mechanism and design criteria.