  A study of searching for doubly charged Higgs $(\delta^{\pm\pm})$ is
performed in two-Higgs-doublet extension of the type-II seesaw model. We find
that a significant mixing effect between singly charged Higgs of Higgs doublet
and of triplet is arisen from the scalar potential. The mixing leads to new
production processes and decay patterns of doubly charged Higgs. With
luminosity of 40 fb$^{-1}$ and collision energy of 13 TeV at the LHC, we show
that $\delta^{\pm\pm}$ with mass below $330$ GeV could be observed at the
$5\sigma$ level. Moreover, for the luminosity of 300 fb$^{-1}$, the observed
mass of $\delta^{\pm\pm}$ could reach up to 450 GeV.
