  Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry,
the mixed-state effect on the low-energy spin dynamics in optimally-doped iron
pnictide superconductors is studied by solving Bogoliubov-de Gennes equations.
Our results of the spin susceptibility at $\mathbf{q}=\mathbf{Q}$ in the
normal, superconducting and mixed states agree qualitatively with recent
neutron scattering experiments. We also propose that the field-induced
intensity change shows different behaviors between the $s_{\pm}$ and s-wave
symmetries in both momentum and real space, thus it can be used to distinguish
these two pairing symmetries.
