  The orbital distribution of the spin fluctuation in the iron-based
superconductors(IBSs) is the key information needed to understand the
magnetism, superconductivity and electronic nematicity in these multi-orbital
systems. In this work, we propose that the resonant inelastic X-ray
scattering(RIXS) technique can be used to probe selectively the spin
fluctuation on different Fe $3d$ orbitals. In particular, the spin fluctuation
on the three $t_{2g}$ orbitals, namely, the $3d_{xz}$, $3d_{yz}$ and the
$3d_{xy}$ orbital, can be selectively probed in the $\sigma\rightarrow\pi'$
scattering geometry by aligning the direction of the outgoing photon in the
$y$, $x$ and $z$ direction. Such orbital-resolved information on the spin
fluctuation is invaluable for the study of the orbital-selective physics in the
IBSs and can greatly advance our understanding on the relation between orbital
ordering and spin nematicity in the IBSs and the orbital-selective pairing
mechanism in these multi-orbital systems.
