The invention relates to a preparation method for a nano-crystalline diamond film field effect transistor, which essentially comprises the steps: putting a pre-treated silicon substrate in the reaction chamber of a microwave plasma chemical vapor deposition device to be taken as a deposition substrate; carrying out the growth of the diamond film in the mixed reaction gas of methane and hydrogen; carrying out hydrogen plasma etching and obtaining the p-typed nano-crystalline diamond film; adopting an ion beam sputtering device and lithography masks technology to manufacture the source, drain and gate electrodes of the field effect transistor on the surface of the p-typed nano-crystalline diamond film. The method can directly manufacture the device without polishing treatment, which has theadvantages of simple manufacturing technique and lower cost, thus being beneficial to the large-scale application of the diamond based devices.