  We study the heavy flavor conserved semi-leptonic decay $B_s\to B\ell\nu$ in
the covariant light front approach. The covariant light front quark model is
used to calculate the transition form factors of $B_s\to B^{(*)}$ as well as
$D_s\to D$, which are consistent with the leading power predictions from the
heavy quark symmetry. The angular distribution analysis on the $B_s\to
B^{(*)}l\bar\nu$ decay is performed by investigating the forward-backward
asymmetry of the lepton. We also study the angular distribution of $B_{s}\to
B^{*}(\to B \gamma)l\bar\nu$ decay both through the lepton forward-backward
asymmetry and the azimuth angle. The branching fractions of $B_s\to Bl\bar\nu$
and $B_s\to B^{*}l\bar\nu$ are at the order $10^{-8}$ and $10^{-9}$,
respectively. The number of $B_s\to B l\bar\nu$ events is estimated to be
$1.76$. The branching fraction of $B_{s}\to B^{*}(\to B \gamma)l\bar\nu$ is at
the order $10^{-11}$, which is calculated by introducing Breit-Wigner
distribution for the intermediate $B^*$.
