To characterize the physicochemical properties of non-covalent SP-AOBE complexes formed between soybean peptides (SPs) and antioxidative phenols from beetroot extracts (AOBEs), the particle size distribution, potential, ultraviolet (UV) absorption characteristics, and antioxidant activity of the complexes were examined. Particle size changes and complexation states during the formation of SP-AOBE complexes were examined using particle size analysis, Zeta potential measurement, and UV absorption spectroscopy. In vitro antioxidant capacity of the complexes was assessed using ABTS+• scavenging activity, hydroxyl radical (•OH) scavenging activity, and ferric reducing power. The results showed a decrease in particle size for the SPs-AOBE complexes compared with SPs or AOBEs alone (P<0.05). With increasing AOBE ratio, particle size initially decreased then increased. The smallest particle size (158.50 nm) and highest absolute Zeta potential value (15.33 mV) for complexes were obtained at an SP-AOBEs mass ratio of 4:1. Upon polyphenol addition, there was a concentration-dependent increase in antioxidant activity measured by ABTS scavenging activity (59.78%), •OH scavenging activity (50.97%), and ferric reducing power (45.62%), peaking at a SP-AOBE mass ratio of 4:1. In conclusion, the SPs-AOBE complexes exhibited uniform particle size distribution, good stability, and enhanced antioxidant activity, thus providing a theoretical foundation for the development and application of antioxidant functional foods.