  The energy injection model is usually proposed to interpret the shallow-decay
phase in Swift GRB X-ray afterglows. However, very few GRBs have simultaneous
signatures of energy injection in their optical and X-ray afterglows. Here, we
report optical observations of GRB 090529A from 2000 sec to $\sim10^6$ sec
after the burst, in which an achromatic decay is seen at both wavelengths. The
optical light curve shows a decay from 0.37 to 0.99 with a break at $\sim10^5$
sec. In the same time interval, the decay indices of the X-ray light curve
changed from 0.04 to 1.2. Comparing these values with the closure relations,
the segment after 3$\times10^{4}$ sec is consistent with the prediction of the
forward shock in an ISM medium without any energy injection. The shallow-decay
phase between 2000 to 3$\times10^{4}$ sec could be due to the external shock in
a wind-type-like medium with an energy injection under the condition of $\nu_o
< \nu_c < \nu_x$. However, the constraint of the spectral region is not well
consistent with the multi-band observations. For this shallow-decay phase,
other models are also possible, such as energy injection with evolving
microphysical parameters, or a jet viewed off-axis,etc.
