  We present an investigation of a coronal cavity observed above the western
limb in the coronal red line Fe X 6374 {\AA} using a telescope of Peking
University and in the green line Fe XIV 5303 {\AA} using a telescope of Yunnan
Observatories, Chinese Academy of Sciences during the total solar eclipse on
2017 August 21. A series of magnetic field models are constructed based on the
magnetograms taken by the Helioseismic and Magnetic Imager onboard the Solar
Dynamics Observatory (SDO) one week before the eclipse. The model field lines
are then compared with coronal structures seen in images taken by the
Atmospheric Imaging Assembly on board SDO and in our coronal red line images.
The best-fit model consists of a flux rope with a twist angle of 3.1$\pi$,
which is consistent with the most probable value of the total twist angle of
interplanetary flux ropes observed at 1 AU. Linear polarization of the Fe XIII
10747 {\AA} line calculated from this model shows a "lagomorphic" signature
that is also observed by the Coronal Multichannel Polarimeter of the High
Altitude Observatory. We also find a ring-shaped structure in the line-of-sight
velocity of Fe XIII 10747 {\AA}, which implies hot plasma flows along a helical
magnetic field structure, in the cavity. These results suggest that the
magnetic structure of the cavity is a highly twisted flux rope, which may erupt
eventually. The temperature structure of the cavity has also been investigated
using the intensity ratio of Fe XIII 10747 {\AA} and Fe X 6374 {\AA}.
