  The relative velocity between baryons and dark matter in the early Universe
can suppress the formation of small-scale baryonic structure and leave an
imprint on the baryon acoustic oscillation (BAO) scale at low redshifts after
reionization. This "streaming velocity" affects the post-reionization gas
distribution by directly reducing the abundance of pre-existing mini-halos
($\lesssim 10^7 M_{\bigodot}$) that could be destroyed by reionization and
indirectly modulating reionization history via photoionization within these
mini-halos. In this work, we investigate the effect of streaming velocity on
the BAO feature in HI 21 cm intensity mapping after reionization, with a focus
on redshifts $3.5\lesssim z\lesssim5.5$. We build a spatially modulated halo
model that includes the dependence of the filtering mass on the local
reionization redshift and thermal history of the intergalactic gas. In our
fiducial model, we find isotropic streaming velocity bias coefficients $b_v$
ranging from $-0.0043$ at $z=3.5$ to $-0.0273$ at $z=5.5$, which indicates that
the BAO scale is stretched (i.e., the peaks shift to lower $k$). In particular,
streaming velocity shifts the transverse BAO scale between 0.121% ($z=3.5$) and
0.35% ($z=5.5$) and shifts the radial BAO scale between 0.167% ($z=3.5$) and
0.505% ($z=5.5$). These shifts exceed the projected error bars from the more
ambitious proposed hemispherical-scale surveys in HI (0.13% at $1\sigma$ per
$\Delta z = 0.5$ bin).
