Abstract: To improve the solubility and stability of lutein (LUT), this study utilized the electrostatic self-assembly method to encapsulate LUT within food-grade protein nanoparticles and composite nanoparticles derived from soybean protein isolates (SPI) and fucoidin (FUD). The impact of the SPI/FUD mass ratio on the formation of ternary composite nanoparticles, known as lutein-soybean protein isolate-fucoidin nanoparticles (LUT-SPI-FUD NPs), was examined. The preparation process underwent optimization through single-factor experiments and the Box-Behnken response surface methodology. Various characterization techniques, including ultraviolet spectroscopy, Fourier-transform infrared spectroscopy, differential scanning calorimetry and X-ray diffraction were employed to delve into the mechanisms governing the formation of these two types of nanoparticles to compare with their solubility, stability, and antioxidant properties. The results showed that LUT-SPI-FUD NPs with SPI/FUD mass ratio of 4:1 had an absolute value of zeta potential greater than 30 mV, a polydispersity index, and a smaller particle size. Under these conditions, the most favorable formulation and process parameters were as follows: A LUT/SPI mass ratio of 1:7.587, SPI mass concentration of 0.83 mg/mL, and a stirring time of 1.493 hours. Characterization analysis verified the ideal particle size and more concentrated distribution of both LUT-SPI NPs and LUT-SPI-FUD NPs, and the binding forces were the interplay of hydrogen bonds, hydrophobic interactions, and electrostatic forces. Solubility, stability, and antioxidant assessments indicated that the conversion of LUT into LUT-SPI NPs and LUT-SPI-FUD NPs substantially increased its solubility in purified water by 350.74 and 432.42 times, respectively. Moreover, both formulations very significant (P<0.01) enhanced the stability and antioxidant capacity of LUT when subjected to ultraviolet light, high temperatures, and oxygen exposure, with LUT-SPI-FUD NPs exhibiting superior performance over LUT-SPI NPs. Consequently, the composite food-grade nanocarrier system, built upon SPI and FUD, represented a more effective technology for augmenting the solubility, stability, and antioxidant properties of LUT, and would offer a promising avenue for future LUT applications.