Abstract: Carbon and nitrogen metabolisms play critical roles in plant growth and development and are closely related to crop yield and grain quality. The stem plays an important role in transporting and storing sugar and nitrogen in a plant. Two typical genotypes of soybean, Nandou 12 and Nandou 20, were used as experimental materials in monoculture and relay intercropping patterns. Under intercropping, soybean was alternatively sown with maize at wide-narrow intervals. The dynamics of soluble sugar and nitrogen contents in the soybean stem and seed were analyzed at different growth stages. In the stem, soluble sugar and nitrogen contents changed in a ‘low-high-low’ trend with stem growth. However, the ratio of soluble sugar to nitrogen contents appeared as a ‘low-high’ trend under monoculture and relay intercropping conditions. Significant differences in stem soluble sugar, nitrogen contents and their ratio under monoculture and intercropping were measured from seedling to full bloom stages (P<0.05), while after that stage no significant differences were measured in either planting pattern (P>0.05). In addition, there were significant correlations between soluble sugar, nitrogen contents and the ratio between the stem and seed from full bloom to seed filling stages, notably for nitrogen content and the ratio, where the maximum correlation coefficient was 0.85. This study thus shows that carbon and nitrogen metabolisms after full bloom stage are closely related to crop yield and grain quality under relay intercropping and monoculture conditions, providing theoretical support for a better understanding of plant transportation of carbon and nitrogen from stem to seed and guidance for effective planting under relay intercropping systems.