Human activities have changed the rate at, and the way in which nitrogen (N) is input from the atmosphere to terrestrial ecosystems, leading to changes in nutrients, and elemental imbalances in forest ecosystems. Many terrestrial ecosystems are currently experiencing increasing inputs of N and P from human activities. In recent years, increased N deposition in forests has occurred due to fossil fuel combustion, N-based fertilizer use, and other activities. It is predicted that N deposition will continue to increase worldwide. On the other hand, forest soils in southern China are more P-limited, because they are highly weathered and leached, and little P is released even from the weathering of primary P-bearing minerals. As a result, P fertilization is widely used in the subtropical forests of southern China. However, relatively little information is available about the effect of P addition on forest growth. Planting density is an important area of research in tree competition, which affects the utilization of plant resources and the acquisition of occupied space. Understanding the relationship between plant density and tree growth would help to estimate and quantify stand development patterns over time. As individual trees in a stand grow in size, individual competition for resources in high-density stands is more intense than in low-density stands. Although there are some studies about the effects of density or N and P additions on tree growth, no studies have systematically examined the effects of N and P additions on plant growth under different planting densities. In this study, the effects of N and P additions on growth and leaf traits of Cinnamomum camphora seedlings under different planting densities were studied, in order to provide a basis for plantation ecosystem management in the subtropical region under global N and P deposition. In this study, NH₄Cl and NaH₂PO₄·2H₂O were selected to simulate atmospheric N and P additions with 4 treatments (CK, N, P and N+P) and seedling density was set at 4 levels (10, 20, 40, and 80 seedings/m²). The results showed that N, P and N+P treatments all increased seedling height and ground diameter of C. camphora seedlings, and that the N+P treatment had the largest effect on seedling growth. N, P and N+P treatments increased the soil and plant analyzer development (SPAD), N and N+P treatments also increased the specific leaf area (SLA), whereas P treatment decreased the SLA. In seedlings treated with N, P, and N+P, seedling height, ground diameter, and SPAD decreased with increasing density, whereas the SLA of seedlings treated with different treatments changed irregularly. The interactive effect of density and N+P addition on SPAD of seedlings was significant.