  GRB 160625B is an extremely bright GRB with three distinct emission episodes.
By analyzing its data observed with the GBM and LAT on board the {\em Fermi}
mission, we find that a multi-color black body (mBB) model can be used to fit
the spectra of initial short episode (Episode I) very well within the
hypothesis of photosphere emission of a fireball model. The time-resolved
spectra of its main episode (Episode II), which was detected with both GBM and
LAT after a long quiet stage ($\sim 180$ seconds) of the initial episode, can
be fitted with a model composing of an mBB component plus a cutoff power-law
(CPL) component. This GRB was detected again in the GBM and LAT bands with a
long extended emission (Episode III) after a quiet period of $\sim 300$
seconds. The spectrum of Episode III is adequately fitted with a CPL plus a
single power-law models, and no mBB component is required. These features may
imply that the emission of three episodes are dominated by distinct physics
process, i.e., Episode I is possible from cocoon emission surrounding the
relativistic jet, Episode II may be from photosphere emission and internal
shock of relativistic jet, and Episode III is contributed by internal and
external shocks of relativistic jet. On the other hand, both X-ray and optical
afterglows are consistent with standard external shocks model.
