Forestry eco-engineering programs in China occupy 721.77 × 10⁴ km², among which plantations have a pivotal role in protecting the fragile ecological environment. Reforestation understory is often ignored because of the simple vertical structure. The importance of light in understory has been discovered. However, how other ecology factors (e.g., soil properties and geographical factors) influence understory composition and stratification remain unclear. In this study, we investigated the effects of understory composition and stratification on environmental factors in black locust plantations. We used systematic clustering analysis based on plant average height to describe understory stratification. The finding of this study was that black locust plantation understory consisted of three levels: (I) a low herbaceous layer (130 cm). Redundancy analysis indicated that soil moisture content and soil total phosphorus content were the largest contributors to the variation in understory vegetation composition. Soil moisture content, altitude, and soil organic carbon content were the largest contributors to the variation in understory stratification. Overall, by analyzing understory stratification and the relationship between soil and geographical factors, we gained a more comprehensive understanding of the interaction between understory and the microenvironment. This is especially important for reforestation management that maintains understory ecology function in the face of global climate change.