Single plant proteins commonly have poor solubility/emulsibility and are associated to high costs as compared with animal protein. To overcome these limitations, we constructed a mung bean (MBP)-whey (WP) double protein nanocomposite system. MBP-WP composite nanoparticles with different compound proportions were prepared using the pH cycle method and were systematically characterized for their microstructure, particle size distribution, zeta potential, fluorescence spectra, Fourier transform infrared (FT-IR) spectra, and emulsifying properties. The particle size of the double protein nanoparticles ranged between 110~200 nm, whereas those of the raw single protein materials were at the micrometer scale. As the proportion of WP increased, significant reductions were observed in surface hydrophobicity, zeta potential (increasing from -32.82 to -24.13 mV), and fluorescence intensity. FT-IR analysis confirmed that no new chemical bonds were formed during nanoparticle preparation. Of note, WP addition enhanced the emulsification activity of the MBP-WP nanoparticles from 3.22~3.56 to 4.85 m2.g-1, while emulsification stability improved significantly from 30.81%~59.41% to 89.68%. These findings demonstrate that MBP-WP nanoparticles prepared via the pH cycle method exhibit homogeneous particle size, stable structure, and significantly improved emulsifying properties. This study establishes a theoretical foundation for applying double protein systems in food delivery applications.