  The leptonic radiative decay $B \to \gamma l\nu$ is of great importance in
the determination of $B$ meson wave functions, and evaluating the form factors
$ F_{V,A}$ are the essential problem on the study of this channel. We computed
next-to-leading power corrections to the form factors within the framework of
PQCD approach, including the power suppressed hard kernel, the contribution
from a complete set of three-particle $B$ meson wave functions up to twist-4
and two-particle off light-cone wave functions, the $1/m_b$ corrections in
heavy quark effective theory, and the contribution from hadronic structure of
photon. In spite of large theoretical uncertainties, the overall power
suppressed contributions decreases about $50\%$ of the leading power result.
The $\lambda_B$ dependence of the integrated branching ratio is reduced after
including the subleading power contributions, thus the power corrections lead
to more ambiguity in the determination of $\lambda_B$ from $B \to \gamma l\nu$
decay.
