  Knowledge about the Lorentz factor and its evolution of relativistic jets in
gamma-ray bursts (GRBs) is crucial to understand their physics. An exact value
of bulk Lorentz factor can be estimated based on a high-energy spectral cutoff,
which may appear in GRBs' prompt emission owing to the absorption of
photon-photon pair production. In this work, we focus on the investigation of
the bulk Lorentz factor evolution of jets in an individual burst. Based on
\textsl{Fermi} observations, we search for the bursts with multiple
$\gamma$-ray pulses characterized by a high-energy spectral cutoff, and nine
GRBs are obtained. Together with the estimation of the pulse duration and
radiation spectrum, the Lorentz factor of jets corresponding to different
pulses in an individual GRB are estimated. It is shown that the Lorentz factor
of jets in an individual GRB fluctuates within a certain range and without a
general trend in these nine GRBs. In addition, the Lorentz factors of the jets
in GRBs~130821A, 160509A and 160625B seem to increase with time. We also study
the relations among $L_{\rm iso }$, $E_{\rm p,z}$, and $\Gamma$ for the pulses
in our sample, which is found to be consistent with that found in previous
works.
