  The rare decays $B_s^0\to D^\pm \pi^\mp$ and $\bar B_s^0\to D^\mp \pi^\pm$
can occur only via annihilation type diagrams in the standard model. In this
paper, we calculate branching ratios of these decays in perturbative QCD
approach ignoring soft final state interaction. From our calculation, we find
that their branching ratios are at $\mathcal{O}(10^{-6})$ with large CP
asymmetry, which may be measured in LHC-b experiment in future.
