Ba0.6Sr0.4TiO3 (BST) thin films were prepared on MgO buffered Pt(111)/Ti/SiO2/Si substrates by pulsed laser deposition. The crystallographic structure, interface characteristics, and dielectric properties of BST thin films are strongly dependent on MgO thickness. BST thin films exhibit (111) preferred orientation when MgO layer is thicker than 5 nm. The MgO layer can mitigate the interdiffusion between BST and Pt, causing the dielectric loss and leakage current of BST thin films to decrease significantly. The dielectric loss, tunability, and the largest figure of merit of BST thin films on 10 nm MgO achieve 0.009, 30%, and 33.4, respectively.