Abstract: The yak casein micelles were modified using succinic anhydride as the reagent.The influences of several factors on the modified degree were studied, the process of succinylated modification was optimized by response surface method and the emulsifying properties were experimented.The results showed that the optimal conditions of succinylation were the ratio of succinic anhydride to casein at 0.6∶1 ( g /g) , temperature of 42℃, reaction time of 49min, pH of 8.8. Under this condition, the modified degree was 86.01% . By succinylation, the emulsifying activity index and emulsion stability index of yak casein increased with the percentage change relative to original casein of 161.64% and 98.54% , respectively. The conditions of succinylated modification on casein were mild and easy to adjust.The degree of modification was higher and the emulsifying properties of modified casein were improved significantly.The results could provide references for the chemical modification of casein.