Internode length and diameter in soybean (Glycine max (L.) Merr.) are closely associated with lodging. The pattern of internode elongation and increase in internode diameter and factors involved were studied in two soybean cultivars, HN48 (tall-stem cultivar) and HN60 (dwarf cultivar), in the growing seasons of 2017 and 2018. Four treatments included natural lighting, shading of the apical meristem, covering of all internodes with aluminium foil, and whole-plant shading with plastic shading nets. When the number of internodes (N) on the main stem was >3, internode N began to elongate. Internode N – 1 exhibited the most rapid elongation, and internode N – 2 elongated slowly. Internode N – 3 stopped elongating, but the increase in internode diameter did not cease as internode elongation stopped. Shading the soybean apical meristem, the stem, and the whole plant all led to internode elongation. Different shading conditions did not alter the pattern of internode elongation. Soybean stem and apical meristem were both light-sensitive tissues. With an increase in shading, internode length increased, whereas internode diameter decreased. Contents of gibberellic acid (GA₃) and salicylic acid in the stem also increased, but abscisic acid content decreased. Shading reduced the size of starch grains but increased the number of osmiophilic granules in the chloroplast. Elevated GA₃ level was the main cause of the changes in internode length and diameter induced by shading. These results suggest that reduction in GA₃ synthesis and enhancement in carbohydrates formation could a strategy for soybean plants to avoid lodging.