  A systematic analysis is presented of the $B_{s}\to K$ form factor $f(q^{2})
$ in the whole range of momentum transfer $q^{2}$, which would be useful to
analyzing the future data on $B_{s}\to K$ decays and extracting $| V_{ub}|$.
With a modified QCD light cone sum rule (LCSR) approach, in which the
contributions cancel out from the twist 3 wavefunctions of $K$ meson, we
investigate in detail the behavior of $f(q^{2})$ at small and intermediate
$q^{2}$ and the nonperturbative quantity $f_{B^{\ast}}g_{B^{\ast}B_{s}K}$
$(f_{B^{\ast}}$ is the decay constant of $B^{\ast}$ meson and
$g_{B^{\ast}B_{s}K}$ the $B^{\ast}B_{s}K$ strong coupling), whose numerical
result is used to study $q^{2}$ dependence of $f(q^{2})$ at large $q^{2}$ in
the single pole approximation. Based on these findings, a form factor model
from the best fit is formulated, which applies to the calculation on $f(q^{2})$
in the whole kinematically accessible range. Also, a comparison is made with
the standard LCSR predictions.
