  The afterglow of GRBs is believed to originate from the synchrotron emission
of shock-accelerated electrons produced by the interaction between the outflow
and the external medium. The accelerated electrons are usually assumed to
follow a power law energy distribution with an index of $p$. Observationally,
although most GRB afterglows have a $p$ larger than 2, there are still a few
GRBs suggestive of a hard ($p<2$) electron spectrum. GRB 091127, with
well-sampled broad-band afterglow data, shows evidence of a hard electron
spectrum and strong spectral evolution, with a spectral break moving from high
to lower energies. The spectral break evolves very fast and cannot be explained
by the cooling break in the standard afterglow model, unless evolving
microphysical parameters are assumed. Besides, the multi-band afterglow light
curves show an achromatic break at around 33 ks. Based on the model of a hard
electron spectrum with an injection break, we interpret the observed spectral
break as the synchrotron frequency corresponding to the injection break, and
the achromatic break as a jet break caused by the jet-edge effect. It is shown
that the spectral evolution and the multi-band afterglow light curves of GRB
091127 can be well reproduced by this model.
