  Fast radio bursts (FRBs) are extra-galactic sources with unknown physical
mechanisms. They emit millisecond-duration radio pulses with isotropic
equivalent energy of $10^{36}\sim10^{41}$ ergs. This corresponds to a
brightness temperature of FRB emission typically reaching the level of
$10^{36}$ K, but can be as high as above $10^{40}$ K for sub-microsecond
timescale structures, suggesting the presence of underlying coherent
relativistic radiation mechanisms. polarization carries the key information to
understand the physical origin of FRBs, with linear polarization usually
tracing the geometric configuration of magnetic fields and circular
polarization probing both intrinsic radiation mechanisms and propagation
effects. Here we show that the repeating sources FRB 20201124A emits $90.9\pm
1.1\%$ circularly polarized radio pulses. Such a high degree of circular
polarization was unexpected in theory and unprecedented in observation in the
case of FRBs, since such a high degree of circular polarization was only common
among Solar or Jovian radio activities, attributed to the sub-relativistic
electrons. We note that there is no obvious correlation between the degree of
circular polarization and burst fluence. Besides the high degree of circular
polarization, we also detected rapid swing and orthogonal jump in the position
angle of linear polarization. The detection of the high degree circular
polarization in FRB 20201124A, together with its linear polarization properties
that show orthogonal modes, place strong constraints on FRB physical
mechanisms, calling for an interplay between magnetospheric radiation and
propagation effects in shaping the observed FRB radiation.
