  MoS$_2$, one of transition metal dichalcogenides (TMDs), has caused a lot of
attentions for its excellent semiconductor characteristics and potential
applications. Here, based on the density functional theory methods, we predict
a novel 2D quantum spin hall (QSH) insulator in the porous allotrope of
monolayer MoS$_2$ (g-MoS$_2$), consisting of MoS$_2$ square and hexagon. The
g-MoS$_2$ has a nontrivial gap as large as 109 meV, comparable with previous
reported 1T'-MoS$_2$ (80 meV), so-MoS$_2$ (25 meV). We demonstrate that the
origin of 2D QSH effect in g-MoS$_2$ originates from the pure d-d band
interaction, different from conventional band inversion between s$-$p, p$-$p or
d$-$p orbitals. Such new polymorph greatly enriches the TMDs family and its
stabilities are confirmed by phonon spectrum analysis. In particular, porous
structure also endows it potential application in efficient gas separation and
energy storage.
