Through adding high methoxyl pectin( HMP) into soy protein isolate( SPI) solution,the changes of particle size,zeta potential and microstructure of SPI / HMP complexes were studied during acidification. The interaction mechanism of SPI at certain concentrations of HMP to improve the stability of acid soy protein beverages was discussed. The results showed that if HMP is less than 0. 2%,at p H above 5. 8 the aggregation of SPI occurred,if HMP concentration is 0. 4% or higher,the aggregation of SPI particles occurred at p H 3. 9 or less. This indicated that the higher concentration of HMP,the lower p H point would be in the aggregation. This may due to HMP absorbed onto the surface of soy protein to prevent aggregation of soy protein particles and maintain the stability of SPI solution.