  The prompt emission mechanism of gamma-ray bursts (GRBs) is still unclear,
and the time-resolved spectral analysis of GRBs is a powerful tool for studying
their underlying physical processes. We performed a detailed time-resolved
spectral analysis of 78 bright long GRB samples detected by Fermi/Gamma-ray
Burst Monitor (GBM). A total of 1490 spectra were obtained and their properties
were studied using a typical Band-shape model. Firstly, the parameter
distribution of the time-resolved spectrum given as follows: the low-energy
spectral index $\alpha \sim -0.72$, high-energy spectral index $\beta \sim
-2.42$, the peak energy $E_{\rm p} \sim 221.69 \,\rm{keV}$, and the energy flux
$F \sim 7.49\times 10^{-6} \rm{\, erg\,cm^{-2}\,s^{-1}}$. More than 80\% of the
bursts exhibit the hardest low-energy spectral index $\alpha_{\rm max}$
exceeding the synchrotron limit (-2/3). Secondly, the evolution patterns of
$\alpha$ and $E_{\rm p}$ were statistically analyzed. The results show that for
multi-pulse GRBs the intensity-tracking pattern is more common than the
hard-to-soft pattern in the evolution of both $E_{\rm p}$ and $\alpha$. The
hard-to-soft pattern is generally shown in single-pulse GRBs or in the initial
pulse of multi-pulse GRBs. Finally, we found a significant positive correlation
between $F$ and $E_{\rm p}$, with half of the samples exhibiting a positive
correlation between $F$ and $\alpha$. We discussed the spectral evolution of
different radiation models. The diversity of spectral evolution patterns
indicates that there may be more than one radiation mechanism occurring in the
gamma-ray burst radiation process, including photospheric radiation and
synchrotron radiation. However, it may also involve only one radiation
mechanism, but more complicated physical details need to be considered.
