  We present an improved analysis of the constraints on the first two
Gegenbauer moments, $a^\pi_2$ and $a^\pi_4$, of the pion's leading-twist
distribution amplitude from a QCD light-cone sum rule analysis of $B\to\pi$
weak transition form factor $f_{+}(q^2)$. Proper chiral current is adopted in
QCD light-cone sum rule so as to eliminate the most uncertain twist-3
contributions to $f_{+}(q^2)$, and then we concentrate our attention on the
properties of the leading-twist pion DA. A nearly model-independent $f_+(q^2)$
as shown in Ref.\cite{pball0} that is based on the spectrum of $B\to\pi l\nu$
decays from BaBar, together with their uncertainties, are adopted as the
standard shape for $f_+(q^2)$ to do our discussion. From a minimum $\chi^2$-fit
and by taking the theoretical uncertainties into account, we obtain
$a^\pi_2(1GeV)=0.17^{+0.15}_{-0.17}$ and $a^\pi_4(1GeV)=-0.06^{+0.20}_{-0.22}$
at the $1\sigma$ confidence level for $m^*_b\in[4.7,4,8] GeV$.
