The formation process of a non-covalent ternary complex comprising soy protein isolate (SPI), soluble starch, and polyacrylic acid (PAA) in aqueous solution was studied based on fluorescence, ultraviolet (UV) absorption, and resonance light scattering (RLS) spectra and the thermal stability of the complex was also investigated. The fluorescence results indicated that SPI and starch could interact with PAA, and the corresponding apparent binding constants were 2.02×105 L/mol and 2.16×103 L/mol, respectively. The addition of PAA to SPI/starch solution not only resulted in changes of fluorescence intensity and the maximum emission wavelength but also generated an apparent common fluorescence emission wavelength, which demonstrated the formation of a new ternary complex. UV and RLS experiments indicated that SPI, starch, and PAA were the basic structural units to produce the ternary complex, and were conjugated through interactions among each other. Moreover, the thermal stability of the ternary complex was investigated by RLS technology and the corresponding kinetic parameters of thermal degradation were calculated based on RLS data, thereby confirming the formation of the ternary complex and the role of PAA in the process of complex formation.