Lodging can reduce grain yield and quality, especially in crops planted at high densities. Force analysis, which relates stem mechanical properties and morphological characteristics under high densities, was used to study lodging in soybean (Glycine max (L.) Merr.). We applied mechanical lodging and examined the relationships between stem bending moment of the breaking force and morphological characteristics in two soybean varieties of contrasting heights grown at four crop population densities (200 000, 300 000, 400 000 and 500 000 plants ha^–1). The experiment was performed in a split-plot design during 2015–16. Measurements were made during the R1–R8 growth stages. Results showed that the full seed stage was the most sensitive period for lodging. Stem strength of the soybean plants increased with growth and development; however, during the R5–R6 stages, the weakened degree of stem strength was greater than the gravity moment. This was the primary reason for the soybeans tending to lodge during this period. In addition, increasing the planting density weakened the mechanical properties significantly. During breeding efforts to improve the soybean lodging resistance, more attention should be paid to strengthening soybean stems. This study also showed that the ratio of stem diameter to plant height could be a new quantitative index for evaluating the lodging resistance of soybeans.