  A long plateau phase and an amazing brightness have been observed in the Xray
afterglow of GRB 060729. This peculiar light curve is likely due to long-term
energy injection in external shock. Here we present a detailed numerical study
on the energy injection process of magnetic dipole radiation from a strongly
magnetized millisecond pulsar and model the multi-band afterglow observations.
It is found that this model can successfully explain the long plateaus in the
observed X-ray and optical afterglow light curves. The sharp break following
the plateaus should be due to the rapid decline of the emission power of the
central pulsar. At an even late time (~5*10^6s), an obvious jet break appears,
which implies a relatively large half opening angle of theta~0.3 for the GRB
ejecta. Due to the energy injection, the Lorentz factor of the outflow is still
larger than two 10^7s post the GRB trigger, making the X-ray afterglow of this
burst detectable by Chandra even 642 days after the burst.
