The growth kinetics of plasma electrolytic oxidation coatings on biodegradable Mg−2% Sr alloy with coarse- and ultrafine-grain structures was evaluated. The parameters characterizing the coating growth kinetic were estimated for the alloys with different structures on the basis of a dissolution/deposition model, and their correlation with the coating properties was demonstrated. The coating morphology and phase composition were related to the protective properties of the alloys and investigated electrochemically in the Ringer’s solution, using uncoated and coated samples. Equivalent circuits modeling “alloy−electrolyte” and “alloy−coating−electrolyte” systems were developed. The PEO coating produced on the alloy with the ultra-fine grain structure displayed the best protective properties.