  We report data analysis results about the outburst evolution and spectral
properties during the hard state of the recently discovered X-ray transient
Swift J1727.8-163 as observed by \emph{Insight}-HXMT and NuSTAR. We find that
the broadband X-ray spectrum of Swift J1727.8-163 is more complex than the most
typical spectral patterns of black hole X-ray binary systems, with not only a
comparatively weaker reflection component but also an additional spectral
continuum component, manifesting itself as a hard X-ray tail beyond the thermal
Comptonization description detectable below 100 keV. This additional component
can be phenomenologically well fitted by adding an extra power-law model with
high energy exponential cutoff in the 2-120 keV energy band. We made an attempt
to explain the broadband X-ray spectral continuum with a thermal/non-thermal
hybrid plasma corona scenario , and find an ultra high compactness parameter
($l_{\rm s}\sim2000$) and a steep non-thermal electron distribution
($\Gamma_{\rm inj}>4$), suggesting the source was accreting with high Eddington
rates and that the electron acceleration mechanism is not very efficient. We
also present a detailed multi-epoch analysis of spectral properties using
\emph{Insight}-HXMT data to investigate the evolution of the key physical
properties regarding the disk and corona during the hard states. No significant
variation is found with the inner disk radius and the coronal temperature
during this time period, and the weak reflection and hard X-ray tail features
are persistent. We discuss the physical implications of our spectral analysis
results in the context of disk-corona relation, particle acceleration, and jet
contribution, during the rise of a black hole X-ray binary in outburst.
