The goal of our research is try to explain the electrophysiological mechanisms of alternation in T-wave of ECG in ultrahigh static magnetic field (SMF). The magneto-hydrodynamics model study shows that ultrahigh SMF can induce reduction in the volume flow rate of the blood in human arota more than 10%, thus may lead to anoxia condition of acute ischemia. Using an ionic-based theoretical model of the cardiac ventricular cell, we simulate transmural heterogeneous suppression of the action potential plateau and action potential duration shortening in case of different anoxia degree. The results demonstrated that anoxia may produce a significant increase in the T-wave amplitude, which may be another mechanism for the influence of ultrahigh SMF to T-wave. This finding is consistent with experimental observation. This study suggests that one should strengthen the safety inspection of ECG in MRI scan and in other application of ultrahigh SMF.