A method for preparing a TiAl alloy thin film, wherein a reaction chamber is provided, in which at least one substrate is placed; an aluminum precursor and a titanium precursor are introduced into the reaction chamber, wherein the aluminum precursor has a molecular structure of a structural formula (I); and the aluminum precursor and the titanium precursor are brought into contact with the substrate so that a titanium-aluminum alloy thin film is formed on the surface of the substrate by vapor deposition. The method solves the problem of poor step coverage ability and the problem of incomplete filling with regard to the small-size devices by the conventional methods. Meanwhile, the formation of titanium-aluminum alloy thin films with the aid of plasma is avoided so that the substrate is not damaged by plasma.