Abstract: In order to solve the problem of poor stability of soybean protein isolate in milk tea system, effect of the coexistent substances in milk tea system (casein, monostearin, butter, tea polyphenols, saccharides, salts, milk powder, black tea powder) on the emulsification (emulsifying activity and emulsifying stability) of soybean protein isolated was investiga in this article. The influence rule of interaction of the substances in milk tea system and soybean protein isolated was discussed through the measurement of Zeta potential and particle size. The results showed that emulsifying activity of soybean protein isolated was minimum when the addition amount of casein, tea polyphenol and lactose was 0.2%, 0.06% and 0.1%, respectively. The average particle size of emulsion was maximum when the addition amount of sodium citrate was 0.02%. The average particle size of emulsion was maximum when the addition amount of magnesium carbonate and skimmed milk powder was 0.01% and 2%, respectively. The emulsifying stability of soybean protein isolate was minimum when the addition amount of lactose was 0.1%. The Zeta potential absolute value of emulsion was minimum when the addition amount of casein, tea polyphenol and lactose was 0.2%, 0.06% and 0.1%, respectively. The Zeta potential absolute value of emulsion was maximum when the addition amount of monoglyceride, carrageenan and magnesium carbonate was 0.15%, 0.02% and 0.01%, respectively. The average particle size of emulsion was maximum when the addition amount of sodium citrate was 0.02% and minimum when the addition amout of magnesium carbonate and skimmed milk powder was 0.01% and 2%, respectively. The different substances in milk tea system influenced the electric double layer structure of the emulsion, so that the emulsibility of soy protein isolate was changed.