Effect of the addition of trace CaO particles (0.1 and 0.3wt. %) into Mg-2Zn-0.5Sr on the microstructure, mechanical properties, and bio-corrosion behaviors was investigated. Microstructures and the composition and distribution of the second phases of Mg-2Zn-0.5Sr-xCaO composites (x=0, 0.1 and 0.3wt. %) were characterized by optical microscope (OM), X-ray diffraction (XRD), electron probe microanalyzer (EPMA), which indicated that the addition of CaO particles can lead to grain refinement and the CaO particles were mostly enriched between the second phases at grain boundaries and the α-Mg matrix. Results of mechanical properties investigated by tensile tests at room temperature show that the addition of CaO particles can improve the strength. However when adding 0.1 wt.% CaO particles, the plasticity decreases, but when adding 0.3 wt.% CaO particles, the plasticity increases. Electrochemical tests show that the corrosion potential (E_corr) of Mg-2Zn-0.5Sr/CaO composites significantly shifts toward a more positive direction when CaO particles were added into Mg-2Zn-0.5Sr. Immersion tests show that the average corrosion rate of Mg-2Zn-0.5Sr is 7.55 mm/year. And when adding 0.1 wt.% CaO, the corrosion rate decreases by about 18.3%, and when adding 0.1 wt.% CaO, the corrosion rate increases by about 52%. Both the microstructure and mechanical properties can be attributed to grain refinement and the mechanical bonding of CaO particles with the second phases and the α-Mg matrix. The bio-corrosion behaviors can be attributed to grain refinement and the formation of table and dense CaHPO₄ protective film due to the reaction of CaO.