The aim of this study was to reflect micro-structure characteristics of dots by means of image processing method for extracting dot structure. Specific to the dot image collected by optical microscope, a method of extracting micro-structure of dots was proposed. In this method， median filter was used to remove the image noise， the principle of maximum inhibition was adopted to judge the edge point， binarization and thinning were used in combination for the final process of the dot structure to find out the realist boundary of dots and printing stocks. In this paper，the simulation testing figure was used to verify the rationality of this method. The dot′s structure by using this method was better than that obtained by traditional edge detection. This method can effectively inhibit background noises, especially for non-overlapping dots whose edge information was retained to the largest degree. And the objective indexes were evaluated as good. The proposed algorithm based on the micro-dots provides a theoretical basis for evaluation of printing quality.