Six-layer Ba0.6Sr0.4TiO3 (BST) films are prepared by an improved sol-gel method. The effects of various preheating methods on morphology and dielectric properties are studied. XRD results reveal that all BST films preferentially grow along the (110) direction and have cubic perovskite structures, and that preheating can obviously enhance crystallization. XPS results show that the preheating can also decrease surface non-perovskite structure and cause perovskite structure to increase markedly. AFM results display that as the preheating number increases, BST films show significant improvement on morphology with surface roughness reducing. The dielectric property measurement exhibits that tunability is remarkably increased with the increase of preheating number. BST film with all layers preheated shows that its tunability exceeds 55% at 20V bias voltage, while BST film with odd number layers preheated presents a moderate capacitance, lower dielectric loss, approximately 50% tunability at 20V bias voltage and significant increase in figure of merit (FOM) with the bias voltage. The excellent combination of dielectric properties can satisfy the application demand of microwave tunable devices. Also, the relevant mechanism of high tunability is discussed.