  GRB 160625B is an extremely-bright outburst with well-monitored afterglow
emission. The geometry-corrected energy is high up to $\sim 5.2\times10^{52}$
erg or even $\sim 8\times 10^{52}$ erg, rendering it the most energetic GRB
prompt emission recorded so far. We analyzed the time-resolved spectra of the
prompt emission and found that in some intervals there were likely
thermal-radiation components and the high energy emission were characterized by
significant cutoff. The bulk Lorentz factors of the outflow material are
estimated accordingly. We found out that the Lorentz factors derived in the
thermal-radiation model are consistent with the luminosity-Lorentz factor
correlation found in other bursts as well as in GRB 090902B for the
time-resolved thermal-radiation components. While the spectral cutoff model
yields much lower Lorentz factors that are in tension with the constraints set
by the electron pair Compoton scattering process. We then suggest that these
spectral cutoffs are more likely related to the particle acceleration process
and that one should be careful in estimating the Lorentz factors if the
spectrum cuts at a rather low energy (e.g., $\sim$ tens MeV). The nature of the
central engine has also been discussed and a stellar-mass black hole is likely
favored.
