  Weakly interacting massive particles, as a major candidate of dark matter
(DM), may directly annihilate or decay into high-energy photons, producing
monochromatic spectral lines in the gamma-ray band. These spectral lines, if
detected, are smoking-gun signatures for the existence of new physics. Using
the 5 years of DAMPE and 13 years of Fermi-LAT data, we search for line-like
signals in the energy range of 3 GeV to 1 TeV from the Galactic halo. Different
regions of interest are considered to accommodate different DM density
profiles. We do not find any significant line structure, and the previously
reported line-like feature at $\sim$133 GeV is also not detected in our
analysis. Adopting a local DM density of $\rho_{\rm local}=0.4\,{\rm
GeV\,cm^{-3}}$, we derive 95% confidence level constraints on the
velocity-averaged cross-section of $\langle{\sigma v}\rangle_{\gamma\gamma}
\lesssim 4 \times 10^{-28}\,{\rm cm^{3}\,s^{-1}}$ and the decay lifetime of
$\tau_{\gamma\nu} \gtrsim 5 \times 10^{29}\,{\rm s}$ at 100 GeV, achieving the
strongest constraints to date for the line energies of 6-660 GeV. The
improvement stems from the longer Fermi-LAT data set used and the inclusion of
DAMPE data in the analysis. The simultaneous use of two independent data sets
could also reduce the systematic uncertainty of the search.
