The invention provides a method for determining degradation amount of SBS modified asphalt. The method comprises the following steps: obtaining the SBS modified asphalt with the doping amount of m known SBS modifiers, and carrying out m aging experiments with different durations so as to obtain m SBS modified asphalt with different aging durations serving as to-be-tested samples; preparing an infrared spectrogram of the m to-be-tested samples; obtaining the butadiene indexes of the m to-be-tested samples; making a relation curve of the butadiene indexes and the aging durations; obtaining the butadiene index of the SBS modified asphalt with the required aging duration according to the relation curve; and based on the doping amount of the SBS modifier in the SBS modified asphalt, the butadiene index of the unaged SBS modified asphalt, and the butadiene index of the SBS modified asphalt with the required aging duration, calculating the degradation amount of the SBS modifier with the required aging duration. According to the method, the degradation amount of the SBS modifier in the aging process of the SBS modified asphalt can be effectively determined, and the detection result is accurate and reliable.