  Filament channel (FC), a plasma volume where the magnetic field is primarily
aligned with the polarity inversion line, is believed to be the pre-eruptive
configuration of coronal mass ejections. Nevertheless, evidence for how the FC
is formed is still elusive. In this paper, we present a detailed study on the
build-up of a FC to understand its formation mechanism. The New Vacuum Solar
Telescope of Yunnan Observatories and Optical and Near-Infrared Solar Eruption
Tracer of Nanjing University, as well as the AIA and HMI on board Solar
Dynamics Observatory are used to study the grow-up process of the FC.
Furthermore, we reconstruct the non-linear force-free field (NLFFF) of the
active region using the regularized Biot-Savart laws (RBSL) and
magnetofrictional method to reveal three-dimension (3D) magnetic field
properties of the FC. We find that partial filament materials are quickly
transferred to longer magnetic field lines formed by small-scale magnetic
reconnection, as evidenced by dot-like H{\alpha}/EUV brightenings and
subsequent bidirectional outflow jets, as well as untwisting motions. The
H{\alpha}/EUV bursts appear repeatedly at the same location and are closely
associated with flux cancellation, which occurs between two small-scale
opposite polarities and is driven by shearing and converging motions. The 3D
NLFFF model reveals that the reconnection takes place in a hyperbolic flux tube
that is located above the flux cancellation site and below the FC. The FC is
gradually built up toward a twisted flux rope via series of small-scale
reconnection events that occur intermittently prior to the eruption.
