  In the framework of type-II two-Higgs-doublet model with a singlet scalar
dark matter $S$, we study the dark matter observables, the electroweak phase
transition, and the gravitational wave signals by such strongly first order
phase transition after imposing the constraints of the LHC Higgs data. We take
the heavy CP-even Higgs $H$ as the only portal between the dark matter and SM
sectors, and find the LHC Higgs data and dark matter observables require $m_S$
and $m_H$ to be larger than 130 GeV and 360 GeV for $m_A=600$ GeV in the case
of the 125 GeV Higgs with the SM-like coupling. Next, we carve out some
parameter space where a strongly first order electroweak phase transition can
be achieved, and find benchmark points for which the amplitudes of
gravitational wave spectra reach the sensitivities of the future gravitational
wave detectors.
