  We aim to review the internal structure and dynamics of the Abell 1351
cluster, shown to host a radio halo with a quite irregular shape. Our analysis
is based on radial velocity data for 135 galaxies obtained at the Telescopio
Nazionale Galileo. We combine galaxy velocities and positions to select 95
cluster galaxy members and analyse the internal dynamics of the whole cluster.
We also examine X-ray data retrieved from Chandra and XMM archives. We measure
the cluster redshift, <z>=0.325, the line-of-sight (LOS) velocity dispersion,
\sigma_v~1500 km/s, and the X-ray temperature, kT~9 keV. From both X-ray and
optical data independently, we estimate a large cluster mass, in the 1--4
$10^{15}$ M$_\odot$ range. We attribute the extremely high value of \sigma_v to
the bimodality in the velocity distribution. We find evidence of a significant
velocity gradient and optical 3D substructure. The X-ray analysis also shows
many features in favour of a complex cluster structure, probably supporting an
ongoing merger of substructures in Abell 1351. The observational scenario
agrees with the presence of two main subclusters in the northern region, each
with its brightest galaxy (BCG1 and BCG2), detected as the two most important
X-ray substructures with a rest-frame LOS velocity difference of \Delta v~2500
km/s (in the rest frame) and probably being in large part aligned with the LOS.
We conclude that Abell 1351 is a massive merging cluster. The details of the
cluster structure allow us to interpret the quite asymmetric radio halo as a
`normal' halo plus a southern relic, strongly supporting a previous suggestion
based only on inspection of radio and preliminary X-ray data.
