  The flat segment lasting $\sim 10^4$ seconds in the X-ray afterglow of
GRB051221A represents the first clear case of strong energy injection in the
external shock of a short GRB afterglow. In this work, we show that a
millisecond pulsar with dipole magnetic field $\sim 10^{14}$ Gauss could well
account for that energy injection. The good quality X-ray flat segment thus
suggests that the central engine of this short burst may be a millisecond
magnetar.
