  When the radio photons propagate through a non-uniform electron density
volume, the plasma lensing effect can induce an extreme magnification to the
observed flux at certain frequencies. Because the plasma lens acts as a
diverging lens, it can extremely suppress the observed flux when aligned with
source. These two properties can theoretically cause a highly magnified Fast
Radio Burst (FRB) to faint or even disappear for a period of time. In this
paper, we interpret that the significant increase in burst counts followed by a
sudden quenching in FRB 20201124A in September 2021 can be attributed to plasma
lensing. Based on the one-dimensional Gaussian lens model, we search for double
main-peak structures in spectra just before its extinction on September 29,
2021. After the de-dispersion and de-scintillation procedures, we find eight
bursts with double main-peaks at stable positions. There are three parameters
in our modelling, the height and width of the one-dimension Gaussian lens and
its distance to the source. We reformulate them as a combined parameter
$\mathrm{P}_0 \propto \left ( \frac{a}{\mathrm{AU}}\right
)\sqrt{\frac{\mathrm{kpc}}{D_{\mathrm{LS}}}
\frac{\mathrm{pc}\;\mathrm{cm}^{-3}}{N_0} }$. The frequency spectra can give an
accurate estimation of $\mathrm{P}_0$ corresponding to $\left (
\frac{a}{\mathrm{AU}}\right )\sqrt{\frac{\mathrm{kpc}}{D_{\mathrm{LS}}}
\frac{\mathrm{pc}\;\mathrm{cm}^{-3}}{N_0} } \approx 28.118$, while the time of
arrival only give a relatively loose constraint on $a^2/D_{\mathrm{LS}}$.
Comparing with the observation dynamic spectra, we suggest that for a plasma
lens in host galaxy, e.g., $D_{\mathrm{LS}}\approx 1\mathrm{kpc}$, the width of
lens can not be larger than $40\mathrm{AU}$. At last, we estimate the relative
transverse motion velocity between the lens and source,
$v\approx98\left(\frac{a}{\mathrm{AU}}\right)\mathrm{km/s}$.
