  The first gravitational-wave event from the merger of a binary neutron star
system (GW170817) was detected recently. The associated short gamma-ray burst
(GRB 170817A) has a low isotropic luminosity ($\sim 10^{47}$ erg s$ ^{-1}$) and
a peak energy $E_{p}\sim 145$ keV during the initial main emission between -0.3
and 0.4 s. The origin of this short GRB is still under debate, but a plausible
interpretation is that it is due to the off-axis emission from a structured
jet. We consider two possibilities. First, since the best-fit spectral model
for the main pulse of GRB 170817A is a cutoff power law with a hard low-energy
photon index ($\alpha =-0.62_{-0.54}^{+0.49} $), we consider an off-axis
photosphere model. We develop a theory of photosphere emission in a structured
jet and find that such a model can reproduce a low-energy photon index that is
softer than a blackbody through enhancing high-latitude emission. The model can
naturally account for the observed spectrum. The best-fit Lorentz factor along
the line of sight is $\sim 20$, which demands that there is a significant delay
between the merger and jet launching. Alternatively, we consider that the
emission is produced via synchrotron radiation in an optically thin region in
an expanding jet with decreasing magnetic fields. This model does not require a
delay of jet launching but demands a larger bulk Lorentz factor along the line
of sight. We perform Markov Chain Monte Carlo fitting to the data within the
framework of both models and obtain good fitting results in both cases.
