Dopamine (DA) was frequently used as a versatile platform to prepare functional coatings for diverse uses due to the excellent surface adhesion property. Our previous research work revealed that the dopamine moieties could enhance the amphiphilicity of polymer and increased the adsorption of polymer at the oil–water interface. This work reported the liquid–liquid interfacial behavior of dopamine modified poly(γ-glutamic acid) polymer (γ-PGA-g-DA) and how the structural transformation of polymer varied with pH value in turn to affect its emulsifying property. The conformation of the γ-PGA-g-DA in aqueous solution was confirmed by means of aqueous electrophoresis techniques, ultraviolet–visible spectrophotometry (UV–vis), dynamic light scattering (DLS), pyrene fluorescence probe and transmission electron microscopy (TEM) methods. The liquid–liquid interfacial behavior was investigated through emulsification experiment. The results revealed that γ-PGA-g-DA had no emulsification below pH 5.0 with a self-assembled aggregate structure. However, γ-PGA-g-DA exhibited excellent emulsifying property above pH 5.0 with a random chain structure. The added tension-activity will offer γ-PGA-g-DA wide applications in the cosmetic, medicine, and food industries due to the biocompatibility and moisture retention of γ-PGA.