  CTB 109 is a middle-aged shell-type SNR with bright thermal X-ray emission.
We reanalyze the GeV $\gamma$-ray emission from CTB 109 using thirteen years of
Pass 8 data recorded by the Fermi Large Area Telescope (Fermi-LAT). The
$\gamma$-ray emission of CTB 109 shows a center bright morphology, which is
well consistent with its thermal X-ray emission rather than the shell-type
structure in the radio band. The spectral analysis shows an evident spectral
curvature at $\sim$ several GeV for the GeV $\gamma$-ray spectrum, which can
naturally explain the lack of TeV $\gamma$-ray emission from CTB 109. Although
either a leptonic or a hadronic model could fit the multi-wavelength
observations of CTB 109, the hadronic model is favored considering its
$\gamma$-ray morphology and the spectral curvature of GeV spectrum. The unusual
$\gamma$-ray spectrum of CTB 109 with other SNRs and the luminosity-diameter
squared relation make CTB 109 to be distinguished both from the young-aged SNRs
with hard GeV $\gamma$-ray spectra and several old-aged SNRs interacting with
molecular clouds.
