  The pulse widths of fast radio bursts are always broadened due to the
scattering of the plasma medium through which the electromagnetic wave passes.
The recorded pulse width will be further affected by the radio telescopes since
the sampling time and the bandwidth cannot be infinitely small. In this study,
we focus on the pulse widths of the 3287 bursts detected from FRB 20121102A as
of October 2023. Various effects such as the scattering broadening, the
redshift broadening and the instrumental broadening are examined. It is found
that the instrumental broadening only contributes a fraction of
$10^{-3}$--$10^{-1}$ to the observed pulse width. The scattering broadening is
even smaller, which constitutes a tiny fraction of $10^{-5}$--$10^{-2}$ in the
observed pulse width. After correcting for these broadenings, the intrinsic
pulse width is derived for each burst. The maximum and minimum pulse widths at
different frequencies are highlighted. Interestingly, both the mean value and
the dispersion range of intrinsic pulse width are found to be inversely
proportional to the square of the central frequency. The intrinsic widths of
most bursts are in a narrow range of 1--10 ms, which leads to a quasi-linear
correlation between the fluence and the peak flux.
