  We study the $B \to \rho$ helicity form factors (HFFs) by applying the
light-cone sum rules up to twist-4 accuracy. The HFF has some advantages in
comparison to the conventionally calculated transition form factors, such as
the HFF parameterization can be achieved via diagonalizable unitarity relations
and etc. At the large recoil point, only the $\rho$-meson longitudinal
component contributes to the HFFs, and we have
$\mathcal{H}_{\rho,0}(0)=0.435^{+0.055}_{-0.045}$ and
$\mathcal{H}_{\rho,\{1,2\}}(0)\equiv 0$. We extrapolate the HFFs to physically
allowable $q^2$-region and apply them to the $B \to \rho$ semileptonic decay.
We observe that the $\rho$-meson longitudinal component dominates its
differential decay width in low $q^2$-region, and its transverse component
dominates the high $q^2$-region. Two ratios $R_{\rm low}$ and $R_{\rm high}$
are used to characterize those properties, and our LCSR calculation gives,
$R_{\rm low}=0.967^{+0.305}_{-0.284}$ and $R_{\rm
high}=0.219^{+0.058}_{-0.070}$, which agree with the BaBar measurements within
errors.
