  To better understand the structures of active-region filaments and the
eruption process, we study an active-region filament eruption in active region
NOAA 11082 in detail on June 22, 2010. Before the filament eruption, the
opposite unidirectional material flows appeared in succession along the spine
of the filament. The rising of the filament triggered two B-class flares at the
upper part of the filament. As the bright material was injected into the
filament from the sites of the flares, the filament exhibited a rapid uplift
accompanying the counterclockwise rotation of the filament body. From the
expansion of the filament, we can see that the filament is consisted of twisted
magnetic field lines. The total twist of the filament is at least 5$\pi$
obtained by using time slice method. According to the morphology change during
the filament eruption, it is found that the active-region filament was a
twisted flux rope and its unwinding motion was like a solar tornado. We also
find that there was a continuous magnetic helicity injection before and during
the filament eruption. It is confirmed that magnetic helicity can be
transferred from the photosphere to the filament. Using the extrapolated
potential fields, the average decay index of the background magnetic fields
over the filament is 0.91. Consequently, these findings imply that the
mechanism of solar filament eruption could be due to the kink instability and
magnetic helicity accumulation.
