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Soy protein isolate/sodium alginate composite films were prepared with stearic acid to improve the water resistance of soy protein isolate/sodium alginate composite films. Effects of different stearic acid additions (0, 2%, 4%, 6%, 8%, 10%) on composite films were evaluated by measuring mechanical properties, water resistance and microstructure of composite films. The results showed that with the 6% and 8% stearic acid addition, elongation at break and water vapor transmittance was significantly decreased, and water content and water solubility were also greatly affected compared with that of the composite film without stearic acid. When the adding amount was 8%, ternary composite films had the lowest water vapor permeability value ((2.95±0.49)g·mm/(m2·h·kPa)) and highest contact angle value (91.68°±9.02°). Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analysis showed that soy protein isolate and sodium alginate formed the network structure through covalent crosslinking, and stearic acid was distributed in the gaps of the network structure. Therefore, when 8% stearic acid was added, the formation of a good network structure made the molecular structure of the composite films denser, and imparted smother surfaces and flatter cross-sections to the composite films, which can improve the water-resistance performance of the composite films. These results showed that the water resistance of soy protein isolate/sodium alginate composite films can be improved by using appropriate stearic acid effectively, which would have important implications in the development of biopolymer-based packaging materials with moisture barrier properties.