Disclosed is a preparation method for a GaAs thin film grown on an Si substrate, said method comprising the following steps: (1) Si (111) substrate cleaning; (2) Si (111) substrate preprocessing; (3) Si (111) substrate oxide film removal; (4) first InxGa1-xAs buffer layer growth; (5) first InxGa1-xAs buffer layer in situ annealing; (6) GaAs buffer layer growth; (7) GaAs buffer layer in situ annealing; (8) second InxGa1-xAs buffer layer growth; (9) second InxGa1-xAs buffer layer in situ annealing; (10) GaAs epitaxial thin film growth. Also disclosed is a GaAs thin film grown on an Si substrate. The GaAs thin film obtained by the present invention has a good crystal quality, an even surface, and a positive promotional significance with regard to the preparation of semiconductor devices, particularly in the field of solar cells.