  A study of searching for doubly charged Higgs $(\delta^{\pm \pm})$ is
performed in two-Higgs-doublet extension of the conventional type-II seesaw
model. We find that a fantastic mixing effect between singly charged Higgs of
Higgs doublet and of triplet is arisen from the scalar potential. The mixing
leads to following intriguing phenomena: (a) the mass splittings in triplet
particles are magnified, (b) QCD processes dominate the production of
$\delta^{\pm \pm}$, and (c) new predominant decay channels of $\delta^{\pm
\pm}$ are $\delta^{\pm \pm} \to W^{\pm^{[*]}} H^{\pm^{(*)}}_{1(2)}$, but not
$\delta^{\pm} \to (\ell^\pm \ell^\pm, W^\pm W^\pm)$ which are usually discussed
in the literature. With luminosity of 40 fb$^{-1}$ and collision energy of 13
TeV, we demonstrate that $\delta^{\pm \pm}$ with mass below $330$ GeV could be
observed at the $5\sigma$ level. Moreover, when the luminosity approaches to
300 fb$^{-1}$, the observed mass of $\delta^{\pm \pm}$ could reach up to 450
GeV.
