Abstract: This study took soybean whey, a typical by-product from soybean processing, as the research material, and completely replaced purified water with soybean whey for the preparation of high-fibre steamed buns. The present work investigated the effects and mechanism of soybean whey on structural characteristics, processing properties, rheological behaviour, moisture distribution and microscopic morphology of high-fibre dough, as well as texture performance, appearance indexes, sensory flavour and functional nutritional components of high-fibre steamed buns. This research aimed to provide theoretical basis and technical data for the high-value recycling of soybean whey and the quality improvement of oat-fibre fortified flour products. High-fibre dough and steamed buns were prepared by incorporating 12% oat fibre into wheat flour. A control group with purified water and an experimental group with soybean whey replacement were arranged. The protein secondary structure, starch X-ray crystal structure, pasting properties, tensile mechanical performance, dynamic rheological characteristics, low-field nuclear magnetic resonance moisture distribution and scanning electron microscope microstructure of dough were systematically determined. Meanwhile, texture parameters including hardness, elasticity and chewiness, specific volume, height-diameter ratio and whiteness of high-fibre steamed buns were measured, and multi-dimensional sensory evaluation was carried out. The contents of polyphenols and soybean isoflavones in finished products were also detected.The results showed that the addition of oat fibre seriously destroyed the molecular cross-linking balance of gluten protein, reduced the proportion of ordered structures such as α-helix and β-sheet, and increased the relative content of β-turn and random coil. Oat fibre promoted the regular arrangement of starch molecular chains and raised the relative crystallinity of starch. It also decreased the pasting viscosity and thermal stability of dough, weakened tensile ductility and rheological elasticity, and competed with gluten and starch for moisture in the system. The distribution proportion of bound water, weakly bound water and free water was changed, the gluten network framework was disrupted, the dough microstructure became loose, and starch granules were disorderly arranged. Consequently, high-fibre steamed buns presented excessive hardness, higher chewiness, lower specific volume, dull colour and uneven internal pores, leading to an overall deterioration of sensory quality. The application of soybean whey effectively alleviated these adverse changes. It promoted the transformation of gluten protein from disordered state to ordered conformation dominated by α-helix, inhibited the regular arrangement of starch molecules and reduced starch relative crystallinity. Soybean whey optimized the pasting temperature, viscosity and retrogradation characteristics of dough, increased tensile resistance and tensile distance, adjusted storage modulus and loss modulus, and enhanced the rheological stability of dough. It redistributed different forms of moisture in the dough, relieved the excessive adsorption of free water caused by oat fibre, and repaired the gaps of gluten network damaged by fibre addition. The starch granules maintained complete morphology and uniform distribution, and the gluten network became continuous and compact. In terms of steamed bun quality, soybean whey significantly reduced the hardness and chewiness of high-fibre steamed buns, increased specific volume, height-diameter ratio and whiteness, and effectively improved appearance collapse, rough taste and insipid flavour. The sensory scores of appearance morphology, internal structure, taste, odour and palatability were significantly improved. In addition, soybean whey remarkably increased the contents of polyphenols and soybean isoflavones, and endowed high-fibre steamed buns with better nutritional value and antioxidant capacity.It was concluded that soybean whey exerted synergistic regulation effects through multiple pathways involving protein conformation, starch crystal structure, moisture migration, rheological properties and microscopic gluten network. It effectively improved the processing adaptability of high-fibre dough and comprehensively enhanced the texture, appearance, sensory and nutritional quality of high-fibre steamed buns. This study realized the environmentally friendly and high-value utilization of soybean processing by-products, and provided a reliable theoretical basis and practical technical route for quality promotion of high-fibre staple food and resource development of agricultural by-products.