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.