Using soy isolate protein (SPI) as raw material and alkenyl succinic anhydride (ASA) as hydrophobic modifier, four kinds of hydrophobic soy isolate protein (ASAn-SPI, the subscript n represented the molar rate of ASA to primary amino group of SPI, n=0.5, 1, 1.5 and 2) with different acylation degree were prepared through controlling the molar rate of ASA to primary amino group of SPI. The structures and properties of ASAn-SPI were characterized by means of primary amino content, FT-IR, UV-vis, fluorescence spectrum and hydrophobic index. The results indicated that long hydrophobic chain was successfully introduced into the SPI molecular framework, the acylation degree and hydrophobic index of ASAn-SPI increased with n and tended to be stable when n exceeded 1.5. Subsequently, ASA1.5-SPI was used in surface sizing of paper and the sizing amount was 0.77 g/m2. It was found that the initial water contact angle of ASA1.5-SPI sized paper was 132?, the retention time of water drop was 53 min. XPS and SEM analysis revealed that ASA1.5-SPI formed a rough hydrophobic layer on paper fiber surface. In addition, mechanical property tests indicated that the tensile strength of ASA1.5-SPI sized paper was increased from 0.6 kN/m to 0.67 kN/m compared with that of base paper.