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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 Hα/EUV brightenings and subsequent bidirectional outflow jets, as well as untwisting motions. The Hα/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.