The preparation technology of BST thin film, electrode preparation, microstructure fabrication and terahertz band measurement method of BST thin film devices are systematically studied. The optimum preparation process of BST thin films is obtained by XRD, AFM and SEM. The electrically tunable frequency selective surface (FSS) in terahertz band is fabricated by electrode fabrication and microstructure fabrication of BSTbased thin film devices. Under condition of the action in electric field, the passband frequency is adjusted from 0.85 THz to 0.87 THz, proving the tuneability of BST thin films in THz band，and the method to determine the dielectric parameters of ferroelectric thin films in THz band is given. The related research can be used to accurately measure the dielectric properties of ferroelectric thin films in THz band, and provide accurate dielectric parameters for the development of functional devices of ferroelectric thin films.