While soil warming has been demonstrated to significantly alter the processes of the nitrogen cycle in forest ecosystems, how leaf-available nitrogen, representing the primary forms of nitrogen absorbed by plants, responds to such thermal alterations remains insufficiently understood. In the present study, a control (CK) group and a soil-warming treatment (W) were set up. The nitrogen contents of nitrate (NO₃⁻-N), ammonium (NH₄⁺-N), and amino acids (AA-N) in previous- and current-year leaves from the upper and lower canopy of Chinese fir were measured under both CK and W conditions. By comparing the differences in available nitrogen distribution across different canopy layers or leaf ages, we aimed to illustrate the effects of soil warming on the allocation of available nitrogen in leaves. It was shown that soil warming can alter the distribution of available nitrogen in Chinese fir leaves, and its impact on leaf AA-N was significantly greater than its impact on inorganic nitrogen. Additionally, the allocation of available nitrogen in Chinese fir under soil warming was also influenced by leaf position and leaf age. Soil warming altered the distribution patterns of available nitrogen in leaves of Chinese fir across different canopy layers or leaf ages, which provides a scientific basis for coniferous tree species to adapt to the thermal environment by regulating available nitrogen allocation.