  The DArk Matter Particle Explorer (DAMPE) is well suitable for searching for
monochromatic and sharp $\gamma$-ray structures in the GeV$-$TeV range thanks
to its unprecedented high energy resolution. In this work, we search for
$\gamma$-ray line structures using five years of DAMPE data. To improve the
sensitivity, we develop two types of dedicated data sets (including the BgoOnly
data which is the first time to be used in the data analysis for the
calorimeter-based gamma-ray observatories) and adopt the signal-to-noise ratio
optimized regions of interest (ROIs) for different DM density profiles. No line
signals or candidates are found between 10 and 300 GeV in the Galaxy. The
constraints on the velocity-averaged cross section for $\chi\chi \to
\gamma\gamma$ and the decay lifetime for $\chi \to \gamma\nu$, both at 95%
confidence level, have been calculated and the systematic uncertainties have
been taken into account. Comparing to the previous Fermi-LAT results, though
DAMPE has an acceptance smaller by a factor of $\sim 10$, similar constraints
on the DM parameters are achieved and below 100 GeV the lower limits on the
decay lifetime are even stronger by a factor of a few. Our results demonstrate
the potential of high-energy-resolution observations on dark matter detection.
