PST thin film doped with Mg was successfully prepared using sol-gel process. The phase structure of the thin film was measured by XRD and the morphology of microstructure of the thin film was observed by SEM. Results showed that the cubic perovskite phase is formed in Mg-doped Pb0.4Sr0.6MgxTi1-xO3-x thin film prepared by sol gel process. The clustered crystallites are distributed and embedded in the base matrix of thin f ilm. However, the contents of the perovskite phase was controlled by both Mg dop ing and heat treatment condition due to formation of intrinsic oxygen vacancy in the perovskite phase. The charge balance between the positive from the intrinsi c oxygen vacancy and the negative from Mg substitution for Ti occurs when small amount of Mg is doped in the perovskite phase. The stable perovskite phase forms easily, and the crystallization of the perovskite phase is hence increased with increasing Mg doping up to x=0.01. The distortion of the perovskite phase struc ture would increase with farther increase of Mg doping in the crystallites above about x=0.01 due to the generation of a lot of oxygen vacancies. Henceforth uns table perovskite phase is formed and the crystallization of the film decreases w ith increasing Mg. Thus highest content of the perovskite phase is reached and t he charge balance between positive and negative occurs in the film when Mg dopin g is x=0.01. Furthermore, the formed crystalline phase may decompose with increa sing Mg content and time of heat treatment. So in the case of high Mg doping the higher the content of Mg and the longer the time of heat treatment, the lower t he content of perovskite phase is contained in the film.