A new application using barium strontium titanate (BST) thin films to fabricate the Pd/BST/Pt heterostructure for hydrogen gas detection has been investigated. To further understand the mechanism of this novel hydrogen sensing properties, impedance spectroscopy of the Pd/BST/Pt has been systematically studied and interpreted satisfactorily based on an equivalent circuit model. The alternate current (AC) electrical data of the extracted parameters and the properties of dielectric permittivity are discussed. The influence of hydrogen on the impedance spectra demonstrates that the charges induced by hydrogen ions mainly occur at the interface rather than in the bulk of the film. These experimental results provide clear evidence for the hydrogen sensing mechanism in the Pd/BST/Pt capacitive device.