  We report the observations of FRB 20220912A using the Five-hundred-meter
Aperture Spherical radio Telescope (FAST). We conducted 17 observations
totaling 8.67 hours and detected a total of 1076 bursts with an event rate up
to 390 hr$^{-1}$. The cumulative energy distribution can be well described
using a broken power-law function with the lower and higher-energy slopes of
$-0.38\pm0.02$ and $-2.07\pm0.07$, respectively. We also report the L band
($1-1.5$ GHz) spectral index of the synthetic spectrum of FRB~20220912A bursts,
which is $-2.6\pm0.21$. The average rotation measure (RM) value of the bursts
from FRB~20220912A is $-0.08\pm5.39\ \rm rad\,m^{-2}$, close to 0 $\rm
rad\,m^{-2}$ and maintain relatively stable over two months. Most bursts have
nearly 100\% linear polarization. About 45\% of the bursts have circular
polarization with SNR $>$ 3, and the highest circular polarization degree can
reach 70\%. Our observations suggest that FRB~20220912A is located in a
relatively clean local environment with complex circular polarization
characteristics. These various behaviors imply that the mechanism of circular
polarization of FRBs likely originates from an intrinsic radiation mechanism,
such as coherent curvature radiation or inverse Compton scattering inside the
magnetosphere of the FRB engine source (e.g. a magnetar).
