The invention provides a polyimide thin film for preparing an artificial graphite film with excellent mechanical properties and a preparation method of the polyimide thin film. The polyimide thin film is prepared from a raw material diamine monomer which comprises 0 to 75 mol ratio of p-phenylenediamine; the raw material dianhydride monomer, comprises 40 percent to 100 mol ratio of of pyromellitic dianhydride; two types of inorganic fillers, including a calcium-containing compound and transition metal oxide, are uniformly dispersed in the polyimide thin film. The polyimide thin film provided by the invention is applicable to a graphitization process of single-sheet, multilayered stacking and continuous coiled materials and can be used for carrying out graphitization at temperature of 2300 DEG C; the crystallization degree is less than 10 percent. The graphite film prepared from the polyimide thin film has an excellent heat conducting property and has excellent mechanical properties in longitudinal and lateral directions of the thin film; the modulus is less than or equal to 2000MPa, the elongation at break is more than or equal to 3 percent, the tensile strength is more than or equal to 40MPa and the difference of the longitudinal and lateral directions is less than 15 percent.