Ba0.6Sr0.4TiO3 (BST) thin films with and without HfO2 buffer layer were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. Dependences of HfO2 thickness on the dielectric property and leakage current of BST thin films were focused. The dielectric constant of BST thin films increased and then decreased with the increase of HfO2 thickness, while the dielectric relaxation was gradually improved. The loss tangent and leakage current under positive bias decreased with the HfO2 thickness increasing. The leakage current analysis based on Schottky emission indicated an improvement of the BST/Pt interface with HfO2 buffer layer. The loss tangent, tunability and figure of merit of optimized HfO2 buffered BST thin film achieved 0.009 8, 21.91% (Emax = 200 kV/cm), 22.40 at 106 Hz, respectively.