The surface strengthening technology of metals includes surface chemical heat-treatment, surface quenching strengthening, surface coating strengthening, surface deformation strengthening and surface severe plastic deformation. This paper focuses on introducing a brand-new surface strengthening technology: surface gradient oxygen-charging strengthening. Different from the traditional carburization and nitriding technologies, the surface gradient oxygen-charging technology is an economic and efficient new way to improve the surface properties of metals. This paper first reviews the development history,application status and shortcomings of the thermal oxidation and oxygen diffusion hardening methods. Then the paper introduces the newly developed surface gradient oxygen-charging technology in detail, and expounds the underlying oxygen-diffusion mechanism and solid solution hardening mechanism. The surface gradient oxygen-charging strengthening technology has the advantages of wide application range, high oxygenation efficiency, significant strengthening and toughening effect, simple processing, low cost, economic, energy saving and good biocompatibility. It has a great application potential as a new way of surface strengthening of metals.