  A model for the twist-3 wave function $\psi_p(x,\mathbf{k_\perp})$ of the
pion has been constructed based on the moment calculation by applying the QCD
sum rules, whose distribution amplitude has a better end-point behavior than
that of the asymptotic one. With this model wave function, the twist-3
contributions including both the usual helicity components
($\lambda_1+\lambda_2=0$) and the higher helicity components
($\lambda_1+\lambda_2=\pm 1$) to the pion form factor have been studied within
the modified pQCD approach. Our results show that the twist-3 contribution
drops fast and it becomes less than the twist-2 contribution at $Q^2\sim
10GeV^2$. The higher helicity components in the twist-3 wave function will give
an extra suppression to the pion form factor. The model dependence of the
twist-3 contribution to the pion form factor has been studied by comparing
three different models. When all the power contributions, which include higher
order in $\alpha_s$, higher helicities, higher twists in DA and etc., have been
taken into account, it is expected that the hard contributions will fit the
present experimental data well at the energy region where pQCD is applicable.
