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A large amount of soybean oil (20%–30% mass ratio of neat asphalt) was selected to partially replace petroleum asphalt. The high-temperature behavior of soybean oil bioasphalt was enhanced by polyphosphoric acid (PPA). Polyphosphoric acid/soybean oil composite modified asphalt (PSMA) was prepared. Temperature sweep, frequency sweep, multiple stress creep recovery, and bending beam rheometer tests were conducted to reveal the rheological behavior. Fourier infrared spectroscopy (FTIR), scanning electron microscope, and fluorescence microscope tests were carried out to reveal its microstructure characteristic. The results proved that polyphosphoric acid obviously increased the high-temperature properties of soybean oil bioasphalt. Compared with the neat asphalt at 0.1 kPa, the recovery rates of 20% bio/6% PPA, 25% bio/6% PPA, and 30% bio/6% PPA increased by 139.2%, 113.5%, and 86.4% respectively. PSMA was less sensitive to temperature and loading frequency, with remarkable elastic recovery and antirutting degeneration ability. PSMA could maintain excellent low-temperature crack resistance at −18°C. PSMA had better rheological properties when soybean oil and PPA were 20%–25% and 4%–6%, respectively. The FTIR tests verified that some new functional groups emerged in PSMA. There were evenly distributed fluorescent materials in the soybean oil bioasphalt under the fluorescence microscope. Under the modification effect of PPA, soybean oil and PPA could be evenly dispersed in PSMA. Laboratory tests verified the superior properties of bioasphalt with high content soybean oil. [ABSTRACT FROM AUTHOR]