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Layered perovskite iridates have been shown to realize a rare J_eff= ½ Mott insulating antiferromagnetic ground state. While the magnetic excitation dispersion of the single layer perovskite iridate Sr₂IrO₄ can be well explained by a classical spin wave modelof an isotropic Heisenberg magnet, the bilayer counterpart Sr₃Ir₂O₇ exhibits a giant magnetic excitation gap, ~90 meV, in the resonant inelastic X-ray scattering (RIXS) spectra whose origin is highly debated in literature. In this talk, we present our recent polarized Raman spectroscopy studies on magnetic excitations in Sr₃Ir₂O₇. Two continuums associated with magnetic excitations are observed, one centering around 170 meV and the other around 100 meV that is close to the magnetic gap reported in RIXS spectra. Through Raman selection rule measurements, we observe that the ~100 meV feature only shows up in the symmetric A_1g channel, while the ~170 meV one is present in both A_1g and B_2g channels. We show that both features originate from two-magnon excitations, by performing symmetry analysis on the spin waves of a bilayer magnet. Our result provides a new insight to the origin of “the giant magnetic gap” in RIXS spectra.