The existence of iron impurity in magnesium (Mg) is conventionally deemed detrimental to the corrosion resistance against the inorganic solutions, especially over its tolerance limit. We deliberately investigated the biodegradation properties of pure Mg with different contents of Fe by immersing in DMEM+10% FBS. Surprisingly, the impurity of Fe, even its content higher than the tolerance limit previously determined by immersion in inorganic solutions such as NaCl, has no negative effect on the degradation of pure Mg. Pure Mg with a high content of Fe impurity even retains good biocompatibility. Detailed microstructural analyses decipher that the severe local micro-galvanic corrosion caused by the Fe impurity meanwhile promotes the formation of compact Ca-rich corrosion product layer on the top surface, protecting the adjacent Mg matrix from being further corroded. The present discovery provides new insights in designing biodegradable Mg alloys, especially pursuing high-level impurity doping without deteriorating the biocompatibility.