Microstructure and bio-corrosion properties of as-cast Mg-Zn-Mn-Y alloys are investigated in detail to reveal the effect of the precipitate on the biocorrosion properties for biomedical application by scanning electronic microscopy electrochemical measurement and immersion tests. Alloy I is composed of primary Mg, Mg7Zn3 and I-phase, while there is only one secondary phase (W-phase) in alloy II. Alloy with W-phase exhibits a higher corrosion resistance and a lower corrosion current density. W-phase acts as a cathode and Mg matrix as an anode at the interface between W-phase and Mg matrix, while I-phase acts as an anode and Mg matrix as a cathode. The standard electrode potentials of secondsry phases are in the following order: W-phase>Mg>I-phase.