Soybean plants regulate morphology and carbohydrate metabolism for growth adaptation when leaves receive low red/far-red (R/Fr) and blue light (B) under shading. In this study, we assessed the effects of shoot morphology and trehalose-6-phosphate (Tre6P) metabolism on soybean growth under different combinations of R/Fr and B light. Soybean plants were treated with low and high light intensities of 200 and 400 µmol m⁻² s⁻¹, respectively, in combination with white light (W) and three levels of R/Fr + B. After 30 days, we conducted morphological measurements and determined Tre6P metabolism in leaves and transcriptomics in stems. Soybean seedlings grown under R₇₆/Fr₄₈ + B₇₆ had higher Tre6P content, lower trehalose 6-phosphate phosphatase (TPP) activity in leaves, and significantly longer second petioles than those grown under R₁₀₀/Fr₁₀ + B₉₀. Tre6P was stored in leaves under R₇₆/Fr₄₈ + B₇₆ treatment, thereby contributing to photosynthate allocation into the second petiole elongation. Compared to stems under R₁₀₀/Fr₁₀ + B₉₀, those under R₇₆/Fr₄₈ + B₇₆ showed enhanced trehalose biosynthesis, which reduced Tre6P content and limited stem elongation to support the stems. Stems under R₅₀/Fr₁₀₀ + B₂₅₀ and R₂₅/Fr₅₀ + B₁₂₅ had significantly elongated first and second petioles compared with those under W₄₀₀ and W₂₀₀, respectively. However, soybean plants under R₂₅/Fr₅₀ + B₁₂₅ showed inhibited stem apex growth and had no third leaf. Soybean plants responded to R/Fr- and B-light exposure, thus regulating Tre6P metabolism to improve petiole elongation when R/Fr decreased from 10 to 2 under low light intensity. Under shade, increasing the B-light intensity (R/Fr = 0.5) inhibited stem apex growth and increased petiole length, to prompt leaves to capture light.