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The early afterglow of a Gamma-ray burst (GRB) can provide criticalinformation on the jet and progenitor of the GRB. The extreme brightness of GRB221009A allows us to probe its early afterglow in unprecedented detail. In thisletter, we report comprehensive observation results of the early afterglow ofGRB 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 20keV to 20 MeV) by \textit{Insight}-HXMT/HE, GECAM-C and \textit{Fermi}/GBM. Wefind that the spectrum of the early afterglow in 20 keV-20 MeV could be welldescribed by a cutoff power-law with an extra power-law which dominates the lowand 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 earlyafterglow phase. By fitting the light curves in different energy bands, we findthat 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 $T_{\rmAG}$=$T_0$+225 s), providing compelling evidence of a jet break. Interestingly,both the pre-break and post-break decay slopes vary with energy, and these twoslopes become closer in the lower energy band, making the break lessidentifiable. Intriguingly, the spectrum of the early afterglow experienced aslight hardening before the break and a softening after the break. Theseresults provide new insights into the understanding of this remarkable GRB.

Observation of GRB 221009A early afterglow in X/$\gamma$-ray energy band