A method was developed to make a quantitative analysis of the microstructure of asphalt modified with styrene-butadiene-styrene block (SBS) copolymer. Moreover, both mixed and branched SBS were employed in the preparation of modified asphalt. By sampling specimens from the top, middle, and bottom, a number of microscope slides were made and 400 times micro images were gained. Binarized with the software of MATLAB, the images then went to the process of distribution test, when four typical distributions were employed to investigate the distribution of modifier spread in asphalt. The distribution characteristic and its parameter were both found. The results show that asphalt phase clearly obeys Poisson distribution, and its parameter,λ, is bond up with the condition SBS performs in. Based on this parameter, indexes can be set up to evaluate the structure of the mixture and its performance as well. Compared with that modified with mixed SBS, the asphalt modified with branched SBS has better performance but worse stability, for it is easier to segregate.