Abstract: Soil structural degradation and water–salt imbalance are two major constraints limiting the improvement of farmland productivity in soda saline–alkali soils of the Songnen Plain. Straw and biochar returning is an important approach for the resource utilization of agricultural wastes and is of great significance for improving the physicochemical properties of saline–alkali soils, enhancing soil quality, and promoting stable and high crop yields. Against this background, a two-year field experiment was conducted in a soda saline–alkali soil region of the Songnen Plain to investigate the effects of combined straw and biochar returning with organic fertilizer application on soil physical properties, water–salt distribution, and soybean yield formation. The experiment was designed to clarify whether the combined application of straw, biochar, and organic fertilizer could simultaneously alleviate soil structural deterioration and water–salt constraints, thereby improving the overall production capacity of saline–alkali farmland.Seven treatments were established, including conventional cultivation (CK), straw application alone (J3B0), and combined straw and biochar application at straw-feedstock equivalent ratios of 2:1 (J2B1) and 1:2 (J1B2), under two fertilization regimes: conventional fertilization (N1) and 70% inorganic fertilizer combined with 30% organic fertilizer (N2). Soil physical properties, soil water and salt-related indicators, and soybean yield and yield components were evaluated during the two-year field experiment. The results showed that combined straw and biochar application with organic fertilizer significantly improved soil physical properties in the plow layer, increased soil volumetric water content, reduced soil salinity and alkalinity, and promoted soybean yield. These responses indicated that the integrated amendment strategy was effective in improving both the structural condition of the soil and the soil water–salt environment, which are important factors affecting crop growth and productivity in soda saline–alkali farmland. Comprehensive index analysis indicated that J2B1N2 exhibited a relatively better overall amelioration effect. Compared with CK, the two-year average total soil porosity under J2B1N2 increased significantly by 9.27% (P<0.05). The soil mean weight diameter, geometric mean diameter, and content of >0.25 mm water-stable aggregates increased significantly by 17.93%, 24.39%, and 8.18%, respectively (P<0.05), demonstrating an improvement in soil aggregate stability and structural quality. Meanwhile, soil pH, electrical conductivity, and exchangeable sodium percentage decreased significantly by 3.19%, 19.58%, and 24.2%, respectively (P<0.05), indicating an effective reduction in soil salinity and alkalinity. In addition, soybean yield increased significantly by 21.9% (P<0.05) compared with CK, showing that the improvement in the soil environment was accompanied by a clear increase in crop productivity.Structural equation modeling further indicated that the amendment treatments mainly promoted soybean yield by improving soil physical structure and regulating the water–salt environment, thereby enhancing yield components and ultimately increasing final yield. This result suggests that the increase in soybean yield was associated with the coordinated improvement of soil structural conditions and the soil water–salt environment rather than with changes in a single soil factor. In conclusion, combined straw and biochar returning with organic fertilizer application can synergistically regulate soil physical structure and the water–salt environment in soda saline–alkali soils and improve soybean yield. These findings provide a reference for reducing limiting factors, improving cultivated land quality, and enhancing farmland productivity in soda saline–alkali soils of Northeast China, and provide a scientific basis for the rational utilization of straw and biochar resources in saline–alkali farmland management.