Based　on　the　Miedema　model　and　Chou　model,　the　activities　of　different　solute　components　in　Mg-Al-Zn,　Mg-Ca-Zn　and　Mg-Al-Ca　ternary　systems　were　calculated.　The　results　show　that　the　variety　of　zinc　content　has　little　influence　on　the　activity　of　Al　or　Ca,　and　the　interaction　of　Zn　and　Al　or　Ca　can　be　neglected　when　the　mass　fraction　of　Zn　is　lower　than　2%　in　the　AZ91　alloy　containing　calcium　(noted　as　Mg-Al-Zn-Ca　system).　Therefore,　the　possible　intermetallic　compounds　in　the　Mg-Al-Zn-Ca　system　can　be　predicted　by　directly　calculating　the　Gibbs　free　energies　of　the　reactions　in　Mg-Al-Ca　system.　The　calculated　Gibbs　free　energies　in　the　Mg-Al-Ca　system　indicate　that　Al2Ca　phase　can　take　priority　of　depositing,　which　agrees　with　the　experimental　results　in　references.　The　consistency　of　calculation　and　experiment　proves　that　the　intermetallic　compounds　in　the　Mg-Al-Zn-Ca　system　can　be　predicted　by　the　Miedema　model　and　Chou　model.