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In order to investigate the key structural features of peach gum polysaccharide (PGP) responsible for its emulsifying function, partial acid hydrolysis of PGP with 0.1 mol/L trifluoroacetic acid was conducted, yielding hydrolysates 30P, 60P, 120P and 180P with different degrees of hydrolysis at different hydrolysis times. The effects of partial acid hydrolysis on the monosaccharide composition and molecular mass of PGP were analyzed by high performance anion-exchange chromatography (HPAEC) and high performance size-exclusion chromatography with a multi-angle laser light scattering detector (HPSEC-MALLS) and the effect of partial acid hydrolysis on the emulsifying function of PGP was investigated by using an oil in water (O/W) emulsion system. Results showed that PGP was an acidic arabinogalactan composed of arabinose, galactose, xylose, mannose, galacturonic acid and glucuronic acid conjugated with a small amount of protein. PGP, whose average molecular mass of 10 980 kU, presented a flexible coil-like chain in 0.1 mol/L NaNO3 solution. As acid hydrolysis progressed, the contents of uronic acid and protein in PGP gradually increased, the molecular mass and degree of branching gradually decreased, and the molecular conformation of the hydrolysates changed from a flexible coil-like to a rigid rod-like structure. The stability of the O/W emulsion prepared using 60P as an emulsifier was significantly improved compared to the other hydrolysates. It was speculated that 60P, whose molecular mass was 1 188 kU and which had reduced degree of branching and higher contents of glycuronic acid-protein conjugate and exhibited a flexible chain-like structure, may be the key structural feature responsible for of the emulsifying function PGP. This study will be helpful to further explain the mechanism action of PGP as an emulsifier.