  We present the analysis of the brightest flare that was recorded in the
\emph{Insight}-HMXT data set, in a broad energy range (2$-$200 keV) from the
microquasar GRS~1915+105 during an unusual low-luminosity state. This flare was
detected by \emph{Insight}-HXMT among a series of flares during 2 June 2019 UTC
16:37:06 to 20:11:36, with a 2-200 keV luminosity of 3.4$-$7.27$\times10^{38}$
erg s$^{-1}$. Basing on the broad-band spectral analysis, we find that the
flare spectrum shows different behaviors during bright and faint epochs. The
spectrum of the flare can be fitted with a model dominated by a power-law
component. Additional components show up in the bright epoch with a hard tail
and in the faint epoch with an absorption line $\sim$ 6.78 keV. The reflection
component of the latter is consistent with an inner disk radius $\sim$ 5 times
larger than that of the former. These results on the giant flare during the
"unusual" low-luminosity state of GRS~1915+105 may suggest that the source
experiences a possible fast transition from a jet-dominated state to a
wind-dominated state. We speculate that the evolving accretion disk and the
large-scale magnetic field may play important roles in this peculiar huge
flare.
