  Motivated by the new observed scalar boson of 126 GeV at ATLAS and CMS,
various phenomena in two-Higgs-doublet model (THDM) are investigated broadly in
the literature. For considering the model that possesses a solution to the
massive neutrinos, we study the simplest extension of conventional type-II
seesaw model to two Higgs doublets. We find that the new interactions in the
scalar potential cause the sizable mixture of charged Higgses in triplet and
doublet. As a result, we have a completely different decay pattern for doubly
charged Higgs ($\delta^{\pm\pm}$), even the vacuum expectation value (VEV) of
Higgs triplet is at GeV level, which is limited by the precision measurement
for $\rho$-parameter. For illustrating the new characters of the model, we
study the influence of new interactions on the new open channels
$\delta^{++}\to ( H^+_1 W^{+^{(*)}}, H^+_1 H^+_1)$ with $H^+_1$ being the
lightest charged Higgs. Additionally, due to the new mixing effect, the triplet
charged Higgs could couple to quarks in the model; therefore, the search for
$\delta^{++}$ via $\delta^{++}\to tb W^+ \to b \bar b W^+ W^+$ by mediated
$H^+_{1}$ becomes significant.
