MgO-based magnetic tunnel junction is a hot issue in the field of spin electronic devices, and its temperature and bias voltage play quite an important role in practical applications. Therefore, it is desiderated to obtain the temperature-bias phase diagram of MgO-based magnetic tunnel junction. This paper develops a theory which is suitable for magnetic tunnel junctions with single crystal barrier. In this theory, the single crystal barrier is regarded as a periodic grating, and the tunneling process is treated by optical diffraction theory, so the coherence of the tunneling electron can be well taken into account. Most importantly, the theory can handle both the temperature effect and bias effect of MgO-based magnetic tunnel junctions. According to the present theory, the temperature-bias phase diagram of MgO-based magnetic tunnel junctions is calculated under different half the exchange splittings, chemical potentials and periodic potentials. The theoretical results show that the extreme phase point of tunneling magnetoresistance (TMR) can move to high temperature region through regulating half the exchange splitting Δof ferromagnetic electrode of MgO-based magnetic tunnel junction. This will be beneficial to the applications of magnetic tunnel junctions at room temperature. Moreover, the chemical potential μcan change the bias corresponding to the maximum phase point of TMR. As is well known, the chemical potential will vary with the material of ferromagnetic electrode. Therefore, if the material of ferromagnetic electrode is chosen with a proper chemical potential, we can obtain a large TMR under high bias voltage. In other words, the output voltage can be considerably increased. This will be favorable for the preparation of high power devices. In addition, it is found that the phase diagram of TMR is significantly dependent on periodic potential v( K _h). As a result, the effects of temperature and bias voltage in the MgO-based magnetic tunnel junctions can be optimized by regulating half the exchange splitting Δ, chemical potential μ, and periodic potential v( K _h). The present work provides a solid theoretical foundation for the applications of MgO-based magnetic tunnel junctions.