Abstract: Soybean whey is a by-product of soybean processing from high-fiber staple food and resources in sustainable agriculture. Taking soybean whey as the research material, here purified water was fully replaced with soybean whey for the preparation of high-fiber steamed buns. A systematic investigation was made to explore the effects and mechanism of soybean whey on structural, processing properties, rheological behavior, moisture distribution, and microscopic morphology of high-fiber dough, as well as texture performance, appearance indexes, sensory flavor, and functional nutritional components of high-fiber steamed buns. The high-value recycling of soybean whey was obtained for the high quality of oat-fiber fortified flour products. High-fiber dough and steamed buns were prepared by incorporating 12% oat fiber into wheat flour. A control group was arranged with purified water, and an experimental group with soybean whey replacement. A series of characterizations was conducted to determine the protein secondary structure, starch X-ray crystal structure, pasting properties, tensile mechanical performance, dynamic rheological properties, low-field nuclear magnetic resonance (NMR) moisture distribution, and scanning electron microscope (SEM) microstructure of dough. Meanwhile, multi-dimensional sensory was evaluated to measure the texture parameters, including hardness, elasticity, and chewiness, specific volume, height-diameter ratio, and whiteness of high-fiber steamed buns. The contents of polyphenols and soybean isoflavones were also detected in finished products. The results showed that the addition of oat fiber seriously destroyed the molecular cross-linking balance of gluten protein, thus reducing the proportion of ordered structures, such as α-helix and β-sheet, with the high relative content of β-turn and random coil. As such, oat fiber also promoted the regular arrangement of starch molecular chains for the high relative crystallinity of starch. There was a decrease in pasting viscosity and thermal stability of dough, as well as tensile ductility and rheological elasticity, indicating the suitable gluten and starch for moisture. The gluten network framework was disrupted to adjust the distribution proportion of bound water, weakly bound water and free water, with loose dough microstructure and disorderly arranged starch granules. Consequently, high-fiber steamed buns presented higher hardness, chewiness, lower specific volume, dull color and uneven internal pores, leading to an overall deterioration of sensory quality. Soybean whey was applied to effectively strengthen high-fiber steamed buns. Gluten protein was transformed from a disordered state into an ordered conformation dominated by α-helix, thus inhibiting the regular arrangement of starch molecules for the low starch relative crystallinity. Soybean whey was used to optimize the pasting temperature, viscosity and retrogradation of dough. Tensile resistance and distance also increased to adjust storage and loss modulus, thereby enhancing the rheological stability of dough. Different forms of moisture were distributed in the dough. The excessive adsorption of free water caused by oat fiber was relieved to repair the gaps of the gluten network damaged by fiber addition. The starch granules maintained complete morphology and uniform distribution, indicating a continuous and compact gluten network. In terms of steamed bun quality, soybean whey reduced the hardness and chewiness of high-fiber steamed buns, whereas there was an increase in the specific volume, height-diameter ratio and whiteness, appearance collapse, rough taste and insipid flavor. In addition, soybean whey remarkably increased the contents of polyphenols and soybean isoflavones. High-fiber steamed buns were obtained with better nutritional value and antioxidant capacity, indicating the high sensory scores of appearance morphology, internal structure, taste, odor and palatability. In conclusion, soybean whey shared synergistic regulation using multiple pathways, particularly with protein conformation, starch crystal structure, moisture migration, rheological properties, and microscopic gluten network. Processing adaptability of high-fiber dough effectively enhanced the texture, appearance, sensory and nutritional quality of high-fiber steamed buns. Theoretical basis and technical data were offered to realize the environmentally friendly and high-value utilization of soybean processing by-products. The findings can provide a practical technical route to promote high-fiber staple food and by-product resources in sustainable agriculture.