To overcome the shortcomings of traditional measurement methods for particle sizes analysis of asphalt emulsion, a new measurement method based on the digital image processing for particle sizes calculation is proposed. This method is realized by three main steps:（1)filling the holes in the asphalt emulsion particles image after thresholding by Otsu algorithm; （2)cutting the linked particles by watershed algorithm in the binary image and eliminating the defective particles by the form factors from edges tracing algorithm; （3)calculating the actual particle sizes and the particle sizes distribution parameters according to the calibrated zoom factor of the microscopic image system and equivalent diameter method. The contrast with laser particle size analyzer shows that the results from image method analysis results are close to that from laser particle size analyzer as the image sample number increases and the standard deviation drops to 0.55 when the images number reaches 100.