  It has recently been shown that through the $\tau\nu$ decay mode the heavy
charged Higgs bosons $H^\pm$ can be discovered at LHC over a vast region of the
parameter space previously thought unaccessible. If the charged Higgs bosons
are discovered in this decay mode and the measurements of its mass and
$\tan\beta$ are made, it allows us to make greater use of the dominant but QCD
background contaminated $bt$ decay mode by cuts suppressing phase space for the
background. In this paper we study the top quark polarisation in the $bt$ decay
mode. When $\tan\beta$ is either moderately large ($\tan\beta\gtrsim9$) or
small ($\tan\beta\lesssim4), the correlation effects can be used as a signal of
$H^\pm$ production. We outline the analysis procedure for various decay
signatures. As an example, we demonstrate the application of our procedure to
the dominant production process $gb\to tH^-$ at LHC. The effect is clearly
observable for $H^\pm$ mass between 200 GeV and 500 GeV in both leptonic and
hadronic channels, but at 500 GeV the boost on the top quark starts to prevent
the reconstruction of invariant masses, and hence the observation of the
correlation effect, in the hadronic decay channel.
