  The early afterglow of a Gamma-ray burst (GRB) can provide critical
information on the jet and progenitor of the GRB. The extreme brightness of GRB
221009A allows us to probe its early afterglow in unprecedented detail. In this
letter, we report comprehensive observation results of the early afterglow of
GRB 221009A (from $T_0$+660 s to $T_0$+1860 s, where $T_0$ is the
\textit{Insight}-HXMT/HE trigger time) in X/$\gamma$-ray energy band (from 20
keV to 20 MeV) by \textit{Insight}-HXMT/HE, GECAM-C and \textit{Fermi}/GBM. We
find that the spectrum of the early afterglow in 20 keV-20 MeV could be well
described by a cutoff power-law with an extra power-law which dominates the low
and high energy bands respectively. The cutoff power-law $E_{\rm peak}$ is
$\sim$ 30 keV and the power-law photon index is $\sim$ 1.8 throughout the early
afterglow phase. By fitting the light curves in different energy bands, we find
that a significant achromatic break (from keV to TeV) is required at $T_0$ +
1246$^{+27}_{-26}$ s (i.e. 1021 s since the afterglow starting time $T_{\rm
AG}$=$T_0$+225 s), providing compelling evidence of a jet break. Interestingly,
both the pre-break and post-break decay slopes vary with energy, and these two
slopes become closer in the lower energy band, making the break less
identifiable. Intriguingly, the spectrum of the early afterglow experienced a
slight hardening before the break and a softening after the break. These
results provide new insights into the understanding of this remarkable GRB.
