The disclosure discloses a method and apparatus for measuring a size of a crystal grain, and a method for fabricating a poly-silicon thin film. The method for measuring the size of the crystal grain includes: obtaining a grain morphology image of a crystalline region of a crystal, and drawing a grain interface diagram according to the grain morphology image; measuring at least one crystal grain in the grain interface diagram, and determining a transverse size and a longitudinal size of each measured crystal grain; and determining a transverse size and a longitudinal size of a crystal grain of the crystal according to the transverse size and the longitudinal size of each measured crystal grain.