In　this　paper,　Mg-3Zn-0.2Ca-0.2Mn　alloys　with　similar　second　phase　content　and　different　grain　sizes　were　obtained　through　the　solid　solution　process,　and　the　alloys　with　different　grain　sizes　were　subjected　to　immersion　and　electrochemical　experiments　in　Hank’s　simulated　body　fluid.　The　effect　of　grain　size　on　the　degradation　behavior　of　biomedical　Mg-3Zn-0.2Ca-0.2Mn　alloy　was　investigated　and　analyzed.　The　electrochemical　experiment　shows　that　the　smaller　the　grain　size　of　the　alloy,　the　smaller　the　corrosion　current　density,　and　the　larger　the　impedance　value.　The　immersion　experiment　and　the　observation　of　the　surface　morphology　shows　that,　with　the　decrease　of　the　grain　size,　the　degradation　rate　of　the　alloy　slows　down,　the　surface　corrosion　pit　size　is　smaller　and　more　uniformly　distributed,　and　the　corrosion　resistance　of　the　alloy　is　better.　In　the　process　of　alloy　degradation,　the　grain　size　affects　the　corrosion　resistance　of　the　alloy　by　affecting　the　density　and　integrity　of　the　degradation　product　film.　The　smaller　the　grain　size　of　the　alloy,　the　denser　and　more　complete　the　product　film　formed　on　the　surface,　and　the　Cl−　adsorbed　on　the　surface　is　not　easy　to　penetrate　into　the　interior,　which　helps　to　improve　the　corrosion　resistance　of　the　alloy.　©　2023　Central　South　University　of　Technology.　All　rights　reserved.