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The intrinsic antiferromagnetic (AFM) topological insulator (TI) MnBi₂Te₄ is expected to possess various exotic quantum states including axion insulator and quantum anomalous Hall effect. When Bi is substituted by Sb, the material crosses a bulk charge neutrality point, from being electron-doped to hole-doped. By applying a magnetic field, it is possible to realize a magnetic Weyl semimetal phase by switching the AFM phase to the FM phase through a meta-magnetic transition at B ~ 6T. Here, we present Shubnikov–de Haas oscillations of MnBi_2-xSb_xTe₄ in a wide range of Sb concentrations close to the charge neutrality point (0.33 ≤ x ≤ 1.21) in the FM state. The evolution of the oscillation frequency and the effective mass shows a good agreement with the Weyl semimetal band-structure of ferromagnetic MnBi₂Te₄ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.