  The PandaX-4T distillation system, designed for the removal of krypton and
radon from xenon, is evaluated for its radon removal efficiency using a
$^{222}$Rn source during the online distillation process. The PandaX-4T dark
matter detector is employed to monitor the temporal evolution of radon
activity. To determine the radon reduction factor, the experimental data of
radon atoms introduced into and bypassed the distillation system is compared.
The results indicate that the PandaX-4T distillation system achieves a radon
reduction factor exceeding 190 at the flow rate of 10 slpm and the reflux ratio
of 1.44. Gas-only online distillation process of a flow rate of 20 slpm is also
conducted without observing significant reduction of radon levels in the
detector. This observation suggests that the migration flow of radon atoms from
the liquid phase to the gas phase is limited, and the flow rate of gas
circulation and duration of the process are insignificant compared to the total
xenon mass of 5.6 tons in the detector. This study provides the experimental
data to support the efficient removal of radon at $\sim$Bq level using the
PandaX-4T distillation system, which is the prerequisite of the radon
background control in the detector. The further operation with higher flow rate
will be applied for the upcoming science run in PandaX-4T.
