This paper presents a comprehensive spectral analysis of the exceptional gamma-ray burst (GRB) 221009A, the brightest ever observed, using data from GECAM-C and Fermi/GBM. The study reveals the presence of emission lines in multiple time intervals, with the central energy of the Gaussian emission line evolving from about 37 MeV to 6 MeV. The line width-to-central energy ratio remains nearly constant at about 10%. Both the central energy and flux of the emission line decay as power laws with indices of -1 and -2, respectively. The observed emission lines are interpreted as arising from blue-shifted electron-positron pair annihilation at 511 keV, emitted by dense clumps traveling with the jet. This scenario allows for the first direct measurement of the jet's bulk Lorentz factor, Γ, which evolves as Γ ∼ t⁻¹ during the prompt emission phase. The flux of the annihilation line in the co-moving frame remains constant, suggesting a stable emission process. These findings provide new insights into the physics of GRBs and relativistic jets, highlighting the importance of spectral line features in understanding the dynamics and composition of jets. The study underscores the significance of multi-instrumental data analysis in probing the extreme conditions of GRBs.This paper presents a comprehensive spectral analysis of the exceptional gamma-ray burst (GRB) 221009A, the brightest ever observed, using data from GECAM-C and Fermi/GBM. The study reveals the presence of emission lines in multiple time intervals, with the central energy of the Gaussian emission line evolving from about 37 MeV to 6 MeV. The line width-to-central energy ratio remains nearly constant at about 10%. Both the central energy and flux of the emission line decay as power laws with indices of -1 and -2, respectively. The observed emission lines are interpreted as arising from blue-shifted electron-positron pair annihilation at 511 keV, emitted by dense clumps traveling with the jet. This scenario allows for the first direct measurement of the jet's bulk Lorentz factor, Γ, which evolves as Γ ∼ t⁻¹ during the prompt emission phase. The flux of the annihilation line in the co-moving frame remains constant, suggesting a stable emission process. These findings provide new insights into the physics of GRBs and relativistic jets, highlighting the importance of spectral line features in understanding the dynamics and composition of jets. The study underscores the significance of multi-instrumental data analysis in probing the extreme conditions of GRBs.