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The Lorentz factor of a relativistic jet and its evolution during the jetexpansion are difficult to estimate, especially for the jets in gamma-raybursts (GRBs). However, it is related to the understanding of jet physics.Owing to the absorption of two-photon pair production($\gamma\gamma{\leftrightarrow}e^+e^-$), a high-energy spectral cutoff mayappear in the radiation spectrum of GRBs. We search such kind of high-energycutoff in GRB 160625B, which is one of the brightest bursts in recent years. Itis found that the high-energy spectral cutoff is obvious for the first pulse inthe second emission episode of GRB 160625B (i.e., $\sim$186-192 s after theburst first trigger), which is smooth and well-shaped. Then, we estimate theLorentz factor and radiation location of the jet shell associated with thefirst pulse in the second emission episode of GRB 160625B. It is found that theradiation location increases with time. In addition, the Lorentz factor remainsalmost constant during the expansion of the jet shell. This reveals that themagnetization of the jet is low or intermediate in the emission region, eventthough the jet could be still Poynting flux dominated at smaller radii to avoida bright thermal component in the emission episode.

Lorentz Factor Evolution of an Expanding Jet Shell Observed in Gamma-ray Burst: Case study of GRB 160625B