The invention provides a method and device for measuring the size of the zinc pattern on the surface of a hot galvanized sheet. The method comprises the following steps: scanning the surface of the hot galvanized sheet to obtain the image data on the surface of the hot galvanized sheet; extracting characteristic wavelength data in the image data based on a preset characteristic wavelength range; determining a characteristic contour combination according to the characteristic wavelength data; determining a zinc pattern grain contour combination according to the characteristic contour combination; measuring the average value of the characteristic length of the zinc pattern grain contour combination, wherein the average value is the size of the zinc pattern. Therefore, the zinc pattern graincontour combination is determined by extracting the characteristic wavelength data in the surface image data of the hot galvanizing sheet, and then the average value of the characteristic length of the zinc pattern grain contour combination is measured, the size of the zinc pattern is determined, and the measurement precision is improved; meanwhile, compared with manual measurement, the measurement efficiency is improved.