Nanocrystalline diamond film is an excellent material for high-tech fields because of its excellent properties. In this paper, research was carried out on the synthesis and deposition mechanism of nanocrystalline diamond thick film in the self-made double bias hot filament chemical vapor (HFCVD) deposited system. Double biases including positive bias on the grid electrode on top of hot filaments and negative bias on the substrate were added to the HFCVD system. During the deposition, nucleation and growth stages were applied alternately to control crystalline size and improve quality. The mechanism of the deposition is discussed in detail in this paper. The positive grid bias increases the active, decomposition and ionization of hydrogen and methane molecules, while negative substrate bias helps positive carbon-containing ions bomb the substrate that leads to the high nucleation density of the diamond. Raman results show that the film prepared has high quality. And SEM results show that the film’s grain size is less than 100nm with thickness of 50μm.