  The detection of the gamma-ray burst (GRB) X-ray emission line is important
for studying the GRB physics and constraining the GRB redshift. Since the
line-like feature in the GRB X-ray spectrum was first reported in 1999, several
works on line searching have been published over the past two decades. Even
though some observations on the X-ray line-like feature were performed, the
significance remains controversial to date. In this paper, we utilize the
down-Comptonization mechanism and present the time evolution of the Fe
K$\alpha$ line emitted near the GRB central engine. The line intensity
decreases with the evolution time, and the time evolution depends on the the
electron density and the electron temperature. In addition, the initial line
with a larger broadening decreases less over time. For instance, when the
emission line penetrates material with the an electron density above $10^{12}$
cm$^{-3}$ at 1 keV, it generally becomes insignificant enough after 100 s for
it not to be detected. The line-like profile deviates from the Gaussian form,
and it finally changes to be similar to a blackbody shape at the time of the
thermal equilibrium between the line photons and the surrounding material.
