  We calculate the $O(\alpha_{ew}m_{t(b)}^{2}/m_{W}^{2})$ and $O(\alpha_{ew}
m_{t(b)}^4/m_W^4)$ supersymmetric electroweak corrections to the process
$H^+\to W^+h$ in the Minimal Supersymmetric Model. These corrections arise from
the virtual effects of the third family quaurks (top and bottom) and squarks
(top-squark and bottom-squark). We find that for $m_{H^+}>200$GeV at low
$\tan\beta(\leq 3)$, the corrections can increase the tree-level decay widths
and the branching radios more than 20% and 40%, respectively.
