  Taiji, a space-based gravitational-wave observatory, consists of three
satellites forming an equilateral triangle with arm length of $3\times 10^6$
km, orbiting around the Sun. Taiji is able to observe the gravitational-wave
standard siren events of massive black hole binary (MBHB) merger, which is
helpful in probing the expansion of the universe. In this paper, we
preliminarily forecast the capability of Taiji for improving cosmological
parameter estimation with the gravitational-wave standard siren data. We
simulate five-year standard siren data based on three fiducial cosmological
models and three models of MBHB's formation and growth. It is found that the
standard siren data from Taiji can effectively break the cosmological parameter
degeneracies generated by the cosmic microwave background (CMB) anisotropies
data, especially for dynamical dark energy models. The constraints on
cosmological parameters are significantly improved by the data combination
CMB+Taiji, compared to the CMB data alone. Compared to the current optical
cosmological observations, Taiji can still provide help in improving the
cosmological parameter estimation to some extent. In addition, we consider an
ideal scenario to investigate the potential of Taiji on constraining
cosmological parameters. We conclude that the standard sirens of MBHB from
Taiji will become a powerful cosmological probe in the future.
