Abstract: Polyphosphoric acid (PPA) is a newly modifier with advantages of low prices and simple process, which has a good application prospect.For study the modification mechanism of PPA modified asphalt, the 0.5%、1%、1.5%、2%(mass ratio of PPA to asphalt) PPA modified asphalt were prepared. Three main indexes test shows that penetration index decreases and softening point increases, moreover ductility decreases as mixing amount of PPA increases, indicating improvement of high-temperature property and a little bit weakness of low-temperature property when asphalt is modified by PPA. The content of asphaltene is increased with the decrease of the content of colloid and the content of aromatics and saturation stay around same test by SARA. The FTIR spectrogram of PPA modified asphalt shows that the whole curve changes and a new absorption peak appears compared with pure asphalt, which reveals the modification mechanism of PPA modified asphalt is chemical modification. Topography images and phase images of PPA modified asphalt were obtained by atomic force microscopy(AFM), which shows that compared with pure asphalt, the area of para-phase is smaller, while the area of peri-phase is larger moreover, and the valley of catana-phase become wider, which means the content of asphaltene increasing and weakening of low temperature anti-cracking performance.