  Adopting the horizon-crossing approximation, we derive the spectral index of
$f_{NL}$ in general N-flation model. Axion N-flation model is taken as a
typical model for generating a large $f_{NL}$ which characterizes the size of
local form bispectrum. We find that its tilt $n_{f_{NL}}$ is negligibly small
when all inflatons have the same potential, but a negative detectable
$n_{f_{NL}}$ can be achieved in the axion N-flation with different decay
constants for different inflatons. The measurement of $n_{f_{NL}}$ can be used
to support or falsify the axion N-flation in the near future.
