  The possible range of $\eta-\eta'$ mixing angle is determined from the
transition form factors $F_{\eta \gamma}(Q^2)$ and $F_{\eta' \gamma}(Q^2)$ with
the help of the present experimental data. For such purpose, the quark-flavor
mixing scheme is adopted and the pseudoscalar transition form factors are
calculated under the light-cone pQCD framework, where the transverse momentum
corrections and the contributions beyond the leading Fock state have been
carefully taken into consideration. We construct a phenomenological expression
to estimate the contributions to the form factors beyond the leading Fock state
based on their asymptotic behavior at $Q^2\to 0$ and $Q^2\to\infty$. By taking
the quark-flavor mixing scheme, our results lead to $\phi= 38.0^{\circ}\pm
1.0^{\circ}\pm 2.0^{\circ}$, where the first error coming from experimental
uncertainty and the second error coming from the uncertainties of the
wavefunction parameters. The possible intrinsic charm component in $\eta$ and
$\eta'$ is discussed and our present analysis also disfavors a large portion of
intrinsic charm component in $\eta$ and $\eta'$, e.g. $|f^c_{\eta'}|\le 50 {\rm
MeV}$.
