  There are few inelastic neutron scattering (INS) reports on the
superconducting single crystals of FeAs-1111 system, even though it was first
discovered in 2008, due to the extreme difficulty in large single crystal
growth. In this paper, we have studied the low-energy spin excitations in the
optimally electron-doped CaFe$_{0.88}$Co$_{0.12}$AsF single crystals with
$T_\mathrm{c}$ = 21 K by INS. The resonance energy of the superconducting spin
resonant mode with $E_\mathrm{r}$ = 12 meV amounts to 6.6
$k_\mathrm{B}$$T_\mathrm{c}$, which constitutes the largest
$E_\mathrm{r}$/$k_\mathrm{B}$$T_\mathrm{c}$ ratio among iron-based
superconductors reported to date. The large ratio implies a strong coupling
between conduction electrons and magnetic excitations in
CaFe$_{0.88}$Co$_{0.12}$AsF. The resonance possesses a magnonlike upward
dispersion along transverse direction due to the anisotropy of spin-spin
correlation length within $ab$ plane in the normal-state, which points to a
spin fluctuation mediated sign-reversed ${s}\mathbf\pm$ wave pairing in
CaFe$_{0.88}$Co$_{0.12}$AsF.
