  HESS J1640-465 is an extended TeV $\gamma$-ray source and its $\gamma$-ray
emission whether from the shell of a supernova remnant (SNR) or a pulsar wind
nebula (PWN) is still under debate. We reanalyze the GeV $\gamma$-ray data in
the field of HESS J1640-465 using eight years of Pass 8 data recorded by the
Fermi Large Area Telescope. An extended GeV $\gamma$-ray source positionally
coincident with HESS J1640-465 is found. Its photon spectrum can be described
by a power-law with an index of $1.42\pm0.19$ in the energy range of 10-500
GeV, and smoothly connects with the TeV spectrum of HESS J1640-465. The
broadband spectrum of HESS J1640-465 can be well fit by a leptonic model with a
broken power-law spectrum of electrons with an exponential cut-off at $\sim$
300 TeV. The spectral properties of HESS J1640-465 are broadly consistent with
the characteristics of other sources identified as PWNe, such as the
correlations between high-energy luminosity ratios and the physical parameters
of pulsar, including spin-down luminosity $\dot{E}$ and characteristic age
$\tau_c$. All these pieces of evidence support that the $\gamma$-ray emission
of HESS J1640-465 may originate from the PWN powered by PSR J1640-4631 rather
than the shell of the SNR G338.3-0.0.
