  Second-order topological insulator, which has (d-2)-dimensional topological
hinge or corner states, has been observed in three-dimensional materials, but
has yet not been observed in two-dimensional system. In this Letter, we
theoretically propose the realization of second-order topological insulator in
the van der Waals heterostructure of CoBr$_2$/Pt$_2$HgSe$_3$/CoBr$_2$.
Pt$_2$HgSe$_3$ is a large gap $\mathbb{Z}_2$ topological insulator. With
in-plane exchange field from neighboring CoBr$_2$, a large band gap above 70
meV opens up at the edge. The corner states, which are robust against edge
disorders and irregular shapes, are confirmed in the nanoflake. We further show
that the second-order topological states can also be realized in the
heterostructure of jacutingaite family $\mathbb{Z}_2$ topological insulators.
We believe that our work will be beneficial for the experimental realization of
second-order topological insulators in van der Waals layered materials.
