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The recent detection of tera-electronvolt (TeV) photons from therecord-breaking gamma-ray burst GRB 221009A during its prompt phase poseschallenges for constraining its Lorentz factor. We re-evaluate the constraintson the jet Lorentz factor considering a two-zone model, wherein the TeV photonsoriginate from the external shock region while the lower energy MeV photonscome from the internal prompt emission region. By properly accounting for theevolution of the MeV photon spectrum and light curve, we calculate the opticaldepth for TeV photons and derive a minimum Lorentz factor about 300. It isconsistent with the afterglow modeling for the TeV emission.

Jet Lorentz factor constraint for GRB 221009A based on the optical depth of the TeV photons