Octenyl succinic anhydride （OSA） was applied to the esterification of purified the original gum arabic （OGA）based on emulsifying agent instead of organic solvent treatment. The process conditions were optimized using one-factor-at-a-time method and responses surface methodology. And we evaluated the effects of esterification on properties of gum arabic. The optimal modified condition were set as follows： OSA， 3.0% （based on the final weight of OGA）； concentration of the OGA， 40%； reaction system pH， 8.5； reaction time， 1.56 h； and reaction temperature， 23 ℃. Under this condition， the graft of the OS in the OSA-GA was 1.97%. Based on the analysis of particle size， ζ-potential and microstructure of emulsion， it was found that the higher apparent viscosity could retard aggregation and stratification of the droplets. And the emulsifying capacity of OSA-GA was improved.