  The quasi-thermal components found in many Fermi gamma-ray bursts (GRBs)
imply that the photosphere emission indeed contributes to the prompt emission
of many GRBs. But whether the observed spectra empirically fitted by the Band
function or cutoff power law, especially the spectral and peak energy ($E_{p}$)
evolutions can be explained by the photosphere emission model alone needs
further discussion. In this work, we investigate in detail the time-resolved
spectra and $E_{p}$ evolutions of photospheric emission from a structured jet,
with an inner-constant and outer-decreasing angular Lorentz factor profile.
Also, a continuous wind with a time-dependent wind luminosity has been
considered. We show that the photosphere spectrum near the peak luminosity is
similar to the cutoff power-law spectrum. The spectrum can have the observed
average low-energy spectral index $\alpha $ $ \sim -1$, and the distribution of
the low-energy spectral index in our photosphere model is similar to that
observed ($-2\lesssim $ $\alpha \lesssim 0$). Furthermore, the two kinds of
spectral evolutions during the decay phase, separated by the width of the core
($\theta _{c}$), are consistent with the time-resolved spectral analysis
results of several Fermi multi-pulse GRBs and single-pulse GRBs, respectively.
Also, for this photosphere model we can reproduce the two kinds of observed
$E_{p}$ evolution patterns rather well. Thus, by considering the photospheric
emission from a structured jet, we reproduce the observations well for the GRBs
best fitted by the cutoff power-law model for the peak-flux spectrum or the
time-integrated spectrum.
