Abstract: In order to study the microstructure and modification mechanisms of polyphosphoric acid (PPA) and styrene-butadiene-styrene block copolymer (SBS) on asphalt, fractionation tests, infrared spectrum tests, fluorescence microscopy tests and differential scanning calorimetry tests were conducted on asphalt binders modified with various dosages (by mass) of PPA and SBS. The results show that the asphaltene content and micelle interaction increase with increasing dosage of PPA, and sol structure of binder gradually changes to sol-gel structure, so that the viscosity is improved. The cross-linking, grafting actions and space mesh structure are strengthened with the addition of PPA, and thus the high temperature storage stability is enhanced. The SBS is reduced to smaller particles and the swelling is improved, so that the modification effectiveness of SBS is further exploited. The mesh structure formed in binder modified with PPA and low dosage of SBS is better than that modified with SBS of high dosage. The glass transition temperature is not significantly influenced by the addition of PPA and it indicates that PPA has little effect on the low temperature performance of binders modified with SBS.