Soybean protein isolate(SPI) was modified by Sodium tripolyphosphate.Phosphorylation reaction conditions were optimized by the orthogonal experiment design and SPSS analysis software.Hydrophobicity determination,FT-IR Spectrum analysis and scanning electron microscope(SEM) analysis were conducted to confirm the phosphorylation of SPI by STP.The results showed that the order of the influence of the four factors on emulsifying properties was: the addition level of STP,the concentration of SPI,reaction time and temperature.The optimal phosphorylation reaction conditions were as follows: the concentration of SPI 13%,the addition level of STP 10%,temperature 40℃ and reaction time 3.5h.Under the optimum conditions,the emulsification activity of SPI modified by phosphorylation was increased from 0.293 to 0.785.Meanwhile,the Emulsification Stability was increased from 17.8 to 26.2.The optimal technology parameters have an instructive significance for the production of SPI with better emulsifying properties.