Ba0.6Sr0.4TiO3 (BST) thin films doped by K (BSTK) from 1 to 20 mol % were fabricated by sol-gel method on a Pt/TiO2/SiO2/Si substrate. Thermal evolutionary process of the Ba0.6Sr0.4TiO3 and (Ba0.6Sr0.4)0.95K0.05TiO3 dry gel was carried out by thermogravimetry and differential thermal analysis system. The structure and surface morphology of BST thin films were investigated as functions of K concentration by x-ray diffraction and atomic force microscopy. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 100 Hz to 1 MHz. The K concentration in BST thin films has a strong influence on the material properties including surface morphology and dielectric and tunable properties. The grain size, surface root-mean-square roughness, dielectric constant, dissipation factor, and tunability all increased with increasing K content up to 7.5 mol % and then decreased with increasing K content from 7.5 to 20 mol % in the BSTK thin films at 1 MHz. The effects of K doping on the microstructure and dielectric and tunable properties of Ba0.6Sr0.4- TiO3 thin films were analyzed. The (Ba0.6Sr0.4)92.5%K7.5%TiO3 thin film exhibited the highest dielectric constant of 1040 and the largest tunability of 73.6%. The dielectric constant, dielectric loss, and tunability of K-doped BST thin films with the optimal K content of 5 mol % were about 971, 0.023, and 69.96%, respectively. In addition, its figure of merit showed a maximum value of approximately 28.52.