  LHAASO J1908+0621 has been recently detected as a source emitting
$\gamma$-rays with energies above 100 TeV, and the multiband observations show
that a break around 1 TeV appears in the $\gamma$-ray spectrum. We have
reanalyzed the GeV $\gamma$-ray properties for the 100 TeV source using 14
years of data recorded by the Fermi Large Area Telescope (Fermi-LAT). The
spectrum in the energy range range 30-500 GeV has an index of 1.50 $\pm$ 0.26,
which is much smaller than that detected in the TeV $\gamma$-rays.
Additionally, the radiation properties of this source are investigated based on
a one-zone time-dependent model. In the model, LHAASO J1908+0621 is associated
with a pulsar wind nebula (PWN) powered by the pulsar PSR J1907$+$0602.
High-energy particles composed of electrons and positrons are injected into the
nebula. Multiband nonthermal emission is produced via synchrotron radiation and
inverse Compton scattering (ICS). Taking the effect of radiative energy losses
and adiabatic cooling into account, the spectral energy distribution from the
model with a broken power-law for the distribution of the injected particles
can explain the detected fluxes in the $\gamma$-ray bands. The results support
that LHAASO J1908+0621 originates from the PWN powered by PSR J1907$+$0602, and
the $\gamma$-rays with energy above 100 TeV are produced by the
electrons/positrons in the nebula via ICS.
