Objective To investigate the effects of κ-carrageenan concentration on its interaction with sodium caseinate and the oil-water interfacial properties of the composite system.Methods Sodium caseinate composite systems with different κ-carrageenan concentrations were prepared. Protein structure, surface hydrophobicity, endogenous fluorescence, zeta potential, particle size, interfacial adsorption behavior, and interfacial dilatational modulus were determined.Results After the addition of κ-carrageenan, the secondary structure of sodium caseinate changed markedly, and the proportion of ordered structures showed a trend of initially increasing and then decreasing. Surface hydrophobicity and endogenous fluorescence intensity exhibited a similar trend. When the addition level of κ-carrageenan was 0.15‰, the contents of α-helix and β-sheet increased from 13.07% and 50.97% to 14.30% and 55.48%, respectively (P<0.05); Surface hydrophobicity decreased from 46 832 to 32 362, and fluorescence intensity decreased from 478 366 to 319 298. With increasing κ-carrageenan addition, the surface electronegativity of the composite system increased and the particle size gradually increased. The ζ-potential decreased from -25.53 mV to -30.05 mV, while the particle size increased from 227.20 nm to 252.80 nm. The adsorption process of the complexes at the oil-water interface gradually slowed, with K_diff decreasing from 0.021 mN/(m·s^1/2) to 0.014 mN/(m·s^1/2). The interfacial dilatational modulus exhibited a trend of initially increasing and then decreasing, increasing from 6.224 mN/m to 8.463 mN/m at a κ-carrageenan addition level of 0.15‰.Conclusion Appropriate addition of κ-carrageenan promoted the formation of a relatively stable composite structure by sodium caseinate and improved its oil-water interfacial properties, with 0.15‰ identified as the optimal addition level.