  The uncertainty of $t\bar{t}$ production cross section measurement at LHC is
at a-few-percent level which still allows the stop pair production
$\tilde{t}\tilde{t}^{*}$ with identical final states $2b+\ell+nj+{E}_{T}$. In
this paper, we attempt to use the existing measurement of $W$-polarization in
top quark decay to improve the distinction between stop and top quark states.
We apply the ATLAS $\chi^2$ method of $W$-polarization measurement in
semi-leptonic $t\bar{t}$ final state to semi-leptonic stop pair samples and
study its prediction. We find that the faked top events from stop mostly
contribute to the left-handed polarized $W$ due to the reconstruction. The
benchmark point with maximal contribution to top events only changes $F_{L}$ by
1%. After comparing with the current experiments, we conclude that the current
measurement of $W$-polarization in $t$ decay cannot exclude the light stop
scenario.
