The uncertainty of tt¯ production cross section measurement at LHC is at a few percent level which still allows the stop pair production t˜t˜* with identical final states 2b+ℓ+nj+ET. 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 χ2 method of W-polarization measurement in semileptonic tt¯ final state to semileptonic 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 FL 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.