  ATLAS and CMS recently show the first results from run 2 of the Large Hadron
Collider (LHC) at $\sqrt{s}=13$ TeV. A resonant bump at a mass of around 750
GeV in the diphoton invariant mass spectrum is indicated and the corresponding
diphoton production cross section is around 3-10 fb. Motivated by the LHC
diphoton excess, we propose that the possible resonance candidate is a Higgs
singlet. To produce the Higgs singlet via gluon-gluon fusion process, we embed
the Higgs singlet in the framework of vector-like triplet quark (VLTQ) model.
As a result, the Higgs singlet decaying to diphoton final state is via VLTQ
loops. Using the enhanced number of new quarks and new Yukawa couplings of the
VLTQs and Higgs singlet, we successfully explain the diphoton production cross
section. We find that the width of the Higgs singlet is below 1 GeV, its
production cross section can be of order of 1 pb at $\sqrt{s}=13$ TeV, and the
branching ratio for it decaying to diphoton is around $0.017$ and is
insensitive to the masses of VLTQs and new Yukawa couplings. We find a strong
correlation between the Higgs Yukawa couplings to $s$-$b$ and $c$-$t$; the
resulted branching ratio for $t \to c h$ can be $1.1\times 10^{-4}$ when the
constraint from $B_s$ oscillation is applied. With the constrained parameter
values, the signal strength for the SM Higgs decaying to diphoton is
$\mu_{\gamma\gamma}< 1.18$, which is consistent with the current measurements
at ATLAS and CMS.
