[Objective] The adaptation mechanism and cost of Robinia pseudoacacia under different site conditions in the loess hilly and gully area of the Loess Plateau were studied to provide support for vegetation restoration in this area.[Methods] By investigating the variations in the functional traits and growth performance of R. pseudoacacia at sites with different slope aspects and positions, we analyzed the main factors affecting the growth of R. pseudoacacia in a heterogeneous environment in the loess hilly and gully area.[Results] ① The leaf nitrogen content, leaf phosphorus content, leaf organic carbon content, root phosphorus content, root organic carbon content, and root tissue density of R. pseudoacacia were higher on sunny slopes than on shady slopes. With a change in the position of the slope from a lower to a higher position, the leaf nitrogen content, leaf phosphorus content, leaf organic carbon content, root phosphorus content, root nitrogen content, specific leaf area, and leaf thickness decreased, while the root organic carbon content and leaf tissue density increased; ② Significant relationships were found between the functional traits of R. pseudoacacia and environmental factors, with the soil factors exerting the most influence. A trade-off between functional traits has been found, not only within the same organ, but also among different organs; ③ The plant height, diameter at breast height (DBH), ground diameter, and crown width of R. pseudoacacia was larger on sunny slopes than on shady slopes and decreased significantly with a change in the slope position from lower to higher. Meanwhile, a significant relationship was also found between the growth and functional traits of R. pseudoacacia.[Conclusion] On sites at lower positions with a south-facing aspect, R. pseudoacacia has a lower adaptation cost and can bring about greater ecological benefits.