  We study the holographic dark energy (HDE) model by using the future
gravitational wave (GW) standard siren data observed from the Einstein
Telescope (ET) in this work. We simulate 1000 GW standard siren data based on a
10-year observation of the ET to make this analysis. We find that all the
cosmological parameters in the HDE model can be tremendously improved by
including the GW standard siren data in the cosmological fit. The GW data
combined with the current cosmic microwave background anisotropies, baryon
acoustic oscillations, and type Ia supernovae data will measure the
cosmological parameters $\Omega_{\rm m}$, $H_0$, and $c$ in the HDE model to be
at the accuracies of 1.28%, 0.59%, and 3.69%, respectively. A comparison with
the cosmological constant model and the constant-$w$ dark energy model shows
that, compared to the standard model, the parameter degeneracies will be broken
more thoroughly in a dynamical dark energy model. We find that the GW data
alone can provide a fairly good measurement for $H_0$, but for other
cosmological parameters the GW data alone can only provide rather weak
measurements. However, due to the fact that the parameter degeneracies can be
broken by the GW data, the standard sirens can play an essential role in
improving the parameter estimation.
