The interaction and synergetic effects of the four corrosive anions (Cl-, HPO42-, HCO3-, and SO42-) on the degradation behavior of the Mg-3Zn-0.5Zr alloy were investigated using some electrochemical tests, such as potentiodynamic polarization, open circuit potential evolution, and electrochemical impedance spectroscopy (EIS). We found that chloride ions induce porous pitting corrosion on the alloy. The corrosion pits expanded on the surface and much deeper. Hydrogen phosphate ions inhibited the degradation rate and restrained the pitting corrosion. The degradation rate of the Mg-3Zn-0.5Zr alloy was accelerated at early time point during immersed in solutions containing hydrogen carbonate ions. However, precipitation of the magnesium carbonate resulted in passivation and the corrosion products totally inhibited the pitting corrosion. Sulfate ions could also corrode the Mg-3Zn-0.5Zr alloy, but for its low concentration in the physiological environment, the effects of sulfate on the degradation rate and corrosion morphology were limited. All of these results develop our knowledge on the degradation mechanism of Mg-3Zn-0.5Zr alloy in the physiological environment.