  The CsI detectors of the High Energy X-ray Telescope of the Hard X-ray
Modulation Telescope (HXMT/CsI) can be used for gamma-ray all sky monitoring
and searching for the electromagnetic counterpart of gravitational wave source.
The instrumental responses are mainly obtained by Monte Carlo simulation with
the Geant4 tool and the mass model of both the satellite and all the payloads,
which is updated and tested with the Crab pulse emission in various incident
directions. Both the Energy-Channel relationship and the energy resolution are
calibrated in two working modes (Normal-Gain mode & Low-Gain Mode) with the
different detection energy ranges. The simulative spectral analyses show that
HXMT/CsI can constrain the spectral parameters much better in the high energy
band than that in the low energy band. The joint spectral analyses are
performed to ten bright GRBs observed simultaneously with HXMT/CsI and other
instruments (Fermi/GBM, Swift/BAT, Konus-Wind), and the results show that the
GRB flux given by HXMT/CsI is systematically higher by $7.0\pm8.8\%$ than those
given by the other instruments. The HXMT/CsI-Fermi/GBM joint fittings also show
that the high energy spectral parameter can be constrained much better as the
HXMT/CsI data are used in the joint fittings.
