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MnBi₂Te₄ brings profound attention because of its rich nature by incorporating topology and magnetism. However, the properties of its surface states in the magnetic phase remain poorly understood. Here we report recent magnetooptical infrared spectroscopy measurements on the intrinsic magnetic topological insulator MnBi₂Te₄. We carry out infrared absorption measurements under magnetic fields up to 34 T and observe an optical absorption in the mid-infrared that shifts to higher energy as a function of increasing magnetic field. The energy of the transition versus magnetic field reflects the behavior of the anomalous Hall effect up to 70T. The transition energy increases through the canted magnetic state of MnBi₂Te₄ and then saturates at 8 T when the system enters a ferromagnetic state. Then, the transition energy further increases when the magnetic field is above 25 T, where the magnetization of Mn antisite defects likely start to flip. In addition, this absorption shows a strong magnetic circular dichroism which likely originates from the bands splitting under the effect of magnetic exchange. Our studies not only reveal the infrared response of MnBi₂Te_a but also provide a picture of how the electronic structure changes through the phase diagram of this material, consistently following the behavior of the magnetization.