The　energetically　stable　tetragonal　nonstoichiometric　La2/3Sr1/3MnO3/BiCoO3　(LSMO/BCO)　interface,　whose　chemical　formula　is　(BiO)(3)(CoO2)(4)/(LaO)(3)(SrO)(1)(MnO2)(3),　was　investigated　by　using　first-principle　calculations.　A　magnetoelectric　coupling　effect,　i.e.,　an　electric　control　of　the　magnetism,　was　approved　by　a　maximum　variation　of　17.9%　in　magnetic　moments　tuned　by　the　electric　polarization　reversal.　The　interfacial　coupling　was　controlled　by　the　polar　shifts　of　the　interfacial　Co　and/or　O　atoms.　A　huge　variation　in　spin-polarization　from　6.3%　to　72.7%　was　achieved　upon　switching　the　electric　polarization.　These　findings　are　useful　for　magnetoelectric　coupled　spintronic　device　applications.