The DAMPE $e^{+}{e}^{-}$ excess at around 1.4 TeV could be explained in the type-II seesaw model with a scalar dark mater $D$ which is stabilized by a discrete $Z_2$ symmetry. The simplest scenario is the annihilation $DD\to H^{++}{H}^{--}$ followed by the subsequent decay $H^{\pm \pm }\to e^{\pm }{e}^{\pm }$, with both the DM and triplet scalars roughly 3 TeV with a small mass splitting. In addition to the Drell-Yan process at future 100 TeV hadron colliders, the doubly charged components could also be produced at lepton colliders like ILC and CLIC in the off shell mode and mediate lepton flavor violating processes $e^{+}{e}^{-}\to {\ell }_i^{\pm }{\ell }_j^{\mp }$ (with $i\ne j$). A wide range of parameter space of the type-II seesaw could be probed, which are well below the current stringent lepton flavor constraints.