Sr0.5Ba0.5-xBixTiO3 (BST) thin films were fabricated on a Pt/SiO2/Si substrate by the sol-gel method. Then follows an investigation of the influences of bismuth (Bi) on the microstructures and the dielectric properties of Sr0.5Ba0.5-xBixTiO3 (BST) thin films. The microstructures of the BST thin films were examined by the XRD and the TEM techniques. Tetragonal perovskite crystal grains were observed in BST thin films. Increasing Bi3+ doping ration in BST will lead to decrease of the grain size. It is found that Bi3+ doping decreases the dielectric loss and improves the frequency dispersion of the BST thin films. Not only is compressed the peak of temperature-dependence of dielectric constant of Bi3+-doped BST thin films but also moves into the low-temperature region. Moreover, the average Curie temperature decreases gradually with the Bi3+ contents increasing.