  Finding the electromagnetic (EM) counterpart of binary compact star merger,
especially the binary neutron star (BNS) merger, is critically important for
gravitational wave (GW) astronomy, cosmology and fundamental physics. On Aug.
17, 2017, Advanced LIGO and \textit{Fermi}/GBM independently triggered the
first BNS merger, GW170817, and its high energy EM counterpart, GRB 170817A,
respectively, resulting in a global observation campaign covering gamma-ray,
X-ray, UV, optical, IR, radio as well as neutrinos. The High Energy X-ray
telescope (HE) onboard \textit{Insight}-HXMT (Hard X-ray Modulation Telescope)
is the unique high-energy gamma-ray telescope that monitored the entire GW
localization area and especially the optical counterpart (SSS17a/AT2017gfo)
with very large collection area ($\sim$1000 cm$^2$) and microsecond time
resolution in 0.2-5 MeV. In addition, \textit{Insight}-HXMT quickly implemented
a Target of Opportunity (ToO) observation to scan the GW localization area for
potential X-ray emission from the GW source. Although it did not detect any
significant high energy (0.2-5 MeV) radiation from GW170817, its observation
helped to confirm the unexpected weak and soft nature of GRB 170817A.
Meanwhile, \textit{Insight}-HXMT/HE provides one of the most stringent
constraints (~10$^{-7}$ to 10$^{-6}$ erg/cm$^2$/s) for both GRB170817A and any
other possible precursor or extended emissions in 0.2-5 MeV, which help us to
better understand the properties of EM radiation from this BNS merger.
Therefore the observation of \textit{Insight}-HXMT constitutes an important
chapter in the full context of multi-wavelength and multi-messenger observation
of this historical GW event.
