Abstract: Increasing planting density can increase grain yield of spring maize, but it leads to mutual shading of leaves in the canopy and elevates the risk of stem lodging. In this study, two light treatments: 50% shading and 100% natural light were applied during the V7-V8 stage under two nitrogen levels:N₀ with 0 kg·hm⁻² and N₂₀₀ with 200 kg·hm⁻². Sampling was collected at the filling stage (R2), milk stage (R3), and dough stage (R4) to measure the morphology, mechanical properties, fresh density, cellulose and lignin content of the third and fifth stem internodes. The results show that shading treatment significantly reduces the diameter of basal stem internodes. Shading significantly decreases puncture strength and crushing strength. During the R2-R4 period, under N₀ conditions, the puncture strength of the third internode decreases by 22.6% - 41.9% and that of the fifth internode by 20.8% - 39.2%; under N₂₀₀ conditions, the puncture strength of the third internode decreases by 26.0% - 47.1% and that of the fifth internode by 18.3% - 35.4%. Similarly, under N₀, the crushing strength of the third internode decreases by 39.4% - 73.6% and that of the fifth internode by 46.8% - 76.9%; under N₂₀₀, the crushing strength of the third internode decreases by 36.3% - 55.9% and that of the fifth internode by 47.5% - 57.7%. Shading significantly reduces stem fresh density, dry weight per unit length, and structural carbohydrate (cellulose and lignin) content, but significantly increases stem moisture content. Correlation analysis reveals that puncture strength and crushing strength are significantly positively correlated with internode diameter, fresh density, and cellulose content, but negatively correlated with internode length. This study elucidates the main reasons of lodging in dense planting maize.