Context: The soybean seed yield in the Huang-Huai-Hai (HHH) region is challenged by high temperatures before blooming. Straw mulching can act to reduce topsoil temperature. However, little is known about whether changes in topsoil temperature contribute to the optimization of soybean root and canopy structure and, ultimately, yield. Objective: The aim of this study is to investigate the effects of straw mulching on soybean topsoil temperature, root growth, and canopy structure in the HHH region, China. Methods: A randomized block design was adopted (2020–2023) in the field, including three straw treatments: straw removing (SR), straw mulching (SM), and straw crushing (SC). Topsoil temperature, root morphology, leaf area index (LAI), light transmittance, canopy photosynthesis, dry matter accumulation, and seed yield of soybean under different treatments were measured. Furthermore, the test results were validated by pot experiment (LT: topsoil cooling, CT: topsoil non-cooling) in 2024. Results: Before soybean blooming, the highest topsoil temperature was 28.47℃ in SR, followed by 27.47℃ in SC and 26.95℃ in SM. Compared to SR and SC, the root length, root surface area, root volume and root dry weight of SM increased by an average of 26.04 %, 27.79 %, 29.13 % and 38.82 %, respectively. Soybean root dry matter weight was significantly positively correlated (P < 0.01) with the LAI and above-ground dry matter accumulation. Compared to SR and SC, Fv/Fm, Y(II), and ETR under SM treatment increased by 8.38 %, 7.94 %, and 7.73 %, respectively. Y(II) of the LT treatment was also significantly (P < 0.05) increased by 17.53 % compared to CT. Among the three treatments, soybean canopy photosynthetic rate and seed yield under SM treatment were, on average, significantly increased by 9.97 %, and 11.87 %, respectively. Furthermore, we identified the LAI characteristics of high-yield soybean canopy: 2.22 0.62 in the lower layer. Conclusion and implications: These findings imply that regulating topsoil temperature through straw mulching optimizes root and canopy development, improving soybean yield. This study provides insights into mitigating heat stress and enhancing sustainable soybean production in warm climates.