Oyster oligopeptides were prepared from shelled oyster meat by two-step enzymatic hydrolysis. The basic physicochemical composition， amino acid composition and relative molecular weight distribution were analyzed. The structure of oyster oligopeptides was characterized by scanning electron microscopy （SEM）， ultraviolet full wavelength scanning （UV） and circular dichroism （CD）. The antioxidant activity in vitro was investigated in four aspects by determining the abilities to scavenge DPPH and ABTS free radicals， oxygen radical absorbance capacity （ORAC） as well as reducing power. The results showed that the protein and peptide contents of oyster oligopeptides were （67.57±0.53）% and （55.34±0.45）%， essential amino acids and hydrophobic amino acids were 22.11% and 21.43%， respectively， and the components with molecular weight less than 1 000 u accounted for 91.16%. The oyster oligopeptides had obvious spherical granules， and had irregular folds and pores on the surface. The maximum absorption peak was at 267 nm wavelength. The contents of α-helix， parallel β-fold， antiparallel β-fold， β-turn and random coil were （5.5±0.2）%， （3.1±0.3）%， （35.7±2.8）%， （19.9±1.1）%， and（34.7±1.9）%， respectively. The IC50 value of oyster oligopeptides to DPPH free radical was about 9.7 mg/mL， and ORAC value was （181.75±7.26） μmol Trolox/g. The ABTS radical scavenging ability of oyster oligopeptides at 5.0 mg/mL was equivalent to that of 0.06 mg/mL VC， and the reducing ability of oyster oligopeptides at 3.0 mg/mL was equivalent to that of 0.01 mg/mL VC.