  Ultra-relativistic jets are believed to play important role in producing
prompt emission and afterglow of gamma-ray burst (GRB), but the nature of the
jet is poorly known owing to the lacking of decisive features observed in the
prompt emission. A series of bright, narrow and regularly-evolving MeV emission
line detected in the brightest-of-all-time GRB 221009A provide unprecedented
opportunity to probe GRB jet physics. The time evolution of the central energy
of the line with power-law index $-1$ is naturally explained by high-latitude
curvature effect. Under the assumption that the line emission is generated in
the prompt emission by $e^\pm$ pair production, cooling and annihilation in the
jet, we can strictly constrain jet physics with observed line emission
properties. We find the radius of the emission region is $r\sim10^{16}$cm. The
narrow line width of $10\%$ implies that pairs cool fast down to
non-relativistic state within a time of tenth of the dynamical time. This
requires a magnetic-field energy density much larger than the prompt gamma-ray
energy density in the jet, implying a magnetic field dominated jet. The
temporal behavior of line flux suggests some angle dependence of line emission.
We also discuss the difficulties of other scenarios to interpret the observed
emission line.
