Inhomogeneous degradation of Mg results in a potential safety risk for the application of its implants. The particular concern is the occurrence of localized accelerated degradation of Mg under the relatively enclosed condition. The present work investigated such a phenomenon using the Mg-4Zn tube in artificial blood plasma. Its occurrence mechanism was explored through elaborate experiments, including the hydrodynamic platform, real-time electrochemical detection, COMSOL simulation, and morphological observations. It is found the deficiency of Ca²⁺ and PO₄³⁺ in the localized solution induced by the excessively high pH value was responsible for its occurrence.