In the framework of the 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 boson H as the only portal between the dark matter and standard model (SM) sectors, and find that the LHC Higgs data and dark matter observables require mS and mH to be larger than 130 GeV and 360 GeV for mA=600  GeV in the case of the 125 GeV Higgs boson 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.