Electronic structure and spin-related state coupling at ferromagnetic material (FM)/MgO (FM=Fe, CoFe, CoFeB) interfaces under biaxial strain are evaluated using the first-principles calculations. The CoFeB/MgO interface, which is su-perior to the Fe/MgO and CoFe/MgO interfaces, can markedly maintain stable and effective coupling channels for majority-spin?1 state under large biaxial strain. Bonding interactions between Fe, Co, and B atoms and the electron transfer between Bloch states are responsible for the redistribution of the majority-spin?1 state, directly infl uencing the coupling effect for the strained interfaces. Layer-projected wave function of the majority-spin?1 state suggests slower decay rate and more stable transport property in the CoFeB/MgO interface, which is expected to maintain a higher tunneling magnetoresistance (TMR) value under large biaxial strain. This work reveals the inteal mechanism for the state coupling at strained FM/MgO interfaces. This study may provide some references to the design and manufacturing of magnetic tunnel junctions with high tunneling magnetoresistance effect.