  A space-based gravitational-wave detector, LISA, consists of a triangle of
three spacecrafts with a separation distance of 2.5 million kilometers in a
heliocentric orbit behind the Earth. Like LISA, Taiji is compose of a triangle
of three spacecrafts with a separation distance of 3 million kilometers in a
heliocentric orbit ahead of the Earth. They are expected to launch in
2030-2035. Assuming a one-year overlap, we propose the LISA-Taiji network in
space to fast and accurately localize the gravitational-wave sources. We use
the Fisher information matrix approach to analyze the sky localization for
coalescing massive black hole binaries. For an equal-mass black hole binary
located at redshift of 1 with a total intrinsic mass of $10^5 M_{\odot}$, the
LISA-Taiji network may achieves about four orders of magnitude improvement on
the event localization region compared to an individual detector. The precision
measurement of sky location from the gravitational-wave signal may completely
identify the host galaxy with low redshifts prior to the final black hole
merger. Such the identification of the host galaxy is helpful for the follow-up
change in electromagnetic emissions of the accretion disk when the massive
black hole binary merges to a single massive black hole, and enables the
coalescing massive black hole binaries to be used as a standard siren.
