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Discovery of emergent magnetic states has drawn a lot of interest in studying geometrically frustrated magnets. We recently studied magnetization plateaus in a layered perovskite Tb₂SrFe₂O₇. Tb₂SrFe₂O₇ has a bilayer perovskite structure (A₃B₂O₇) with Tb and Sr both at A-sites alternately ordered along the c-axis. Different from the multiferroic Ca₂SrFe₂O₇ that hosts the polar crystal structure and the magnetic ordered state with canted Fe moments, Tb₂SrFe₂O₇ has the non-polar structure symmetry of P42/mnm and the antiferromagnetic structure for the Fe-sublattice below 600 K. The magnetization plateaus were observed below the second transition at 15 K. With the field applied along c-axis, three plateaus were observed. Single crystal neutron diffraction revealed that the magnetic transition at 15 K is from magnetic order of Tb-sublattice accompanied with the spin reorientation of Fe-sublattice. It was also proved that the order of 2-in-2-out spin structure is due to magnetic coupling with the Fe-sublattice. In this presentation, I will show the evolution of the spin structure with temperature and magnetic field and disclose the nature of the magnetization plateaus in Tb₂SrFe₂O₇.