Abstract: To explore the physiological responses and adaptation mechanisms of P. massoniana Lamb. inoculated with ectomycorrhizal fungi (EMF) under increasing nitrogen deposition, one-year-old P. massoniana seedlings were inoculated with EMF and stimulated by nitrogen deposition (0, 30, 60, 90 kg N·hm⁻²·a⁻¹, respectively). The effects of nitrogen application and inoculation on the growth and photosynthetic characteristics of P. massoniana seedlings were then studied. Results showed that: (1) Under the same nitrogen concentration, both kinds of EMF promoted the growth and photosynthesis of P. massoniana seedlings, and inoculation had significant effects on plant height, growth, chlorophyll b, P_n, G_s, C_i, and T_r. The growth promoting effects of both EMF differed under different nitrogen concentrations. (2) Within a certain range of nitrogen deposition, increasing nitrogen promoted P. massoniana seedling growth. Diameter, aboveground dry weight, and underground dry weight increased with the increase in nitrogen concentration. (3) Under interactive treatment, the growth and photosynthetic indices of P. massoniana seedlings increased, among which plant height, growth, and chlorophyll b increased significantly, indicating that nitrogen application and EMF inoculation promoted P. massoniana growth. In general, EMF inoculation promoted the growth, photosynthesis, and productivity of P. massoniana under a certain range of nitrogen deposition. Under a background of increasing nitrogen deposition in the future, nitrogen application can promote the growth of mycorrhizal P. massoniana, providing a basis for the artificial afforestation of P. massoniana as a dominant tree species in the Three Gorges Reservoir area.