  We investigate the prospects of detecting gravitational waves from coalescing
massive black hole binaries in the Universe with the TianQin observatory, a
space-based gravitational wave interferometer proposed to be launched in the
2030s. To frame the scientific scope of the mission, in this paper, we carry
out a preliminary estimation of the signal-to-noise ratio, detection rate and
parameter estimation precision of massive black hole binaries detectable by
TianQin. In order to make our results as robust as possible, we consider
several models of the growth history of massive black holes, exploring the
effect of some key astrophysical prescriptions as well the impact of the
employed computational methods. In the most optimistic model, TianQin can
detect as many as approximately 60 mergers per year. If TianQin detects a
merger at redshift of 15, it will be capable of estimating its luminosity
distance to within an accuracy of 10%; for a nearby event at redshift
approximately 2, TianQin can issue early warnings 24 hours before coalescence,
with a timing accuracy of around three hours and a sky localization ability of
approximately 80 deg$^2$, thus enabling multi-messenger observations.
