Studying the dynamics of filaments at pre-eruption phase can shed light on the precursor of eruptive events .
Such studies in high-resolution ( in the order of 0.1 ” ) are highly desirable yet very rare so far .
In this work , we present a detailed observation of a pre-eruption evolution of a filament obtained by the 1.6 m New Solar Telescope ( NST ) at Big Bear Solar Observatory ( BBSO ) .
One end of the filament is anchored at the sunspot in NOAA active region ( AR ) 11515 , which is well observed by NST H \alpha off-bands four hours before till one hour after the filament eruption .
A M1.6 flare is associated with the eruption .
We observed persistent downflowing materials along the H \alpha multi-threaded component of the loop towards the AR end during the pre-eruption phase .
We traced the trajectories of plasma blobs along the H \alpha threads and obtained the plane-of-sky velocity of 45 km s ^ { -1 } on average .
We further estimated the real velocities of the downflows and the altitude of the filament by matching the observed H \alpha threads with magnetic field lines extrapolated from a nonlinear force-free field ( NLFFF ) model .
Observation of chromospheric brightenings ( BZs ) at the footpoints of the falling plasma blobs is also presented in the paper .
The lower limit of the kinetic energy per second of the downflows through the BZs is found to be \sim 10 ^ { 21 } erg .
Larger FOV observations from BBSO full disk H \alpha images show that the AR end of the filament started ascending four hours before the flare .
We attribute the observed downflows at the AR end of the filament to the draining effect of the filament rising prior to its eruption .
During the slow-rise phase , the downflows continuously drained away \sim 10 ^ { 15 } g mass from the filament over a few hours , which is believed to be essential for the instability at last , and could be an important precursor of eruptive events .