The features of the abnormal microstructure during the austenitic decomposition in Fe–1.1%C alloy under different magnetic field intensity have been investigated by optical microscopy. It was found that the high magnetic field considerably affects the abnormal microstructure in Fe–1.1%C alloy. The high magnetic field increases the area fraction and width of the abnormal microstructure through remarkably decreasing the Gibbs energy needed for the ferrite transformation. Simultaneously, due to the fact that the Ae1 line shifts to high temperature side with the increase of magnetic field intensity, the abnormal microstructure transformation is imposed to stop and the pearlite transformation begins. Therefore, the area fraction of the abnormal microstructure firstly increases and then decreases with the increase of the magnetic field intensity.