  A resonance in the diphoton channel with invariant mass $m_{\gamma \gamma} =
750$ GeV was claimed by the ATLAS and CMS collaborations at the run-2 LHC with
$\sqrt{s}=13$ TeV. In this paper, we explain this diphoton excess as a
pseudo-scalar singlet, which is produced at the LHC through gluon fusion with
exotic scalar quarks running in the loop. We point out the scalar singlet might
trigger a two-step electroweak phase transition, which can be strongly first
oder. By assuming there are CP violations associated with interactions of the
scalar quarks, the model is found to be able to generate adequate baryon
asymmetry of the Universe through the electroweak baryogenesis mechanism.
Constraints on the model are studied.
