The content SBS plays an important role on the performances of its modified asphalt. Traditional methods which were based on the differences of performances and molecular weight couldn’t quantify SBS content accurately. On the basis of FTIR analysis of SBS, the characteristic peak combination of modified asphalt that could be possibly used for quantitative analysis was summarized. According to theory of Lambert-Beer’s law, a relationship of absorbance ratio and content of mass was deduced. Considering the physical property of SBS polymer modified asphalt and coefficient of variation of specimens, the standard method to prepare SBS polymer modified asphalt for FTIR test was established. Then, regression coefficients were tested for significances combining with theoretical equation and linear programming method, the optimum characteristic peak ratio combination for quantification was designated. Finally, a reasonably accurate and precise method to quantify SBS content is achieved, and the author expected the research could provide the guidance for quantification of polymer content in polymer modified asphalt through this paper.