  We study the accretion on to the neutron star in Be/X-ray binaries, using a
3D SPH code and the data imported from a simulation by \citet{oka2} for a
coplanar system with a short period ($P_{\rm orb}=24.3 \rm{d}$) and a moderate
eccentricity $(e=0.34)$, which targeted the Be/X-ray binary 4U 0115+63. For
simplicity, we adopt the polytropic equation of state. We find that a
time-dependent accretion disc is formed around the neutron star regardless of
the simulation parameters. In the long term, the disc evolves via a two-stage
process, which consists of the initial developing stage and the later developed
stage. The developed disc is nearly Keplerian. In the short term, the disc
structure modulates with the orbital phase. The disc shrinks at the periastron
passage of the Be star and restores its radius afterwards. The accretion rate
on to the neutron star is also phase dependent, but its peak is broader and
much lower than that of the mass-transfer rate from the Be disc, unless the
polytropic exponent is as large as 5/3. Our simulations show that the truncated
Be disk model for Be/X-ray binaries is consistent with the observed X-ray
behaviour of 4U 0115+63.
