Nitrogen deposition usually increases plant N:P stoichiometry and potentially intensifies phosphorus limitations for plant aboveground or belowground growth. However, the effects of P addition on N:P stoichiometry, aboveground biomass (AGB), and belowground biomass (BGB) of the subtropical Chinese fir (Cunninghamia lanceolata) with N deposition remains poorly understood. A 1-year study was conducted to examine the effects of N deposition (0, 30, and 60 kg N ha⁻¹ yr⁻¹; N0, N30, and N60) and P application (0, 20, and 40 mg kg⁻¹; P0, P20, and P40) on biomass production and N:P stoichiometry in Chinese fir seedlings. N60 alone significantly increased the total biomass; AGB; BGB; N concentrations in the leaves, branches, and roots; and root N:P ratio. P addition alone significantly increased total biomass and AGB. P40 significantly increased P concentration in the leaves, branches, stems, and roots but did not influence N concentration and decreased the N:P ratio. Compared to that with N60 alone, P40 combined with N60 significantly increased the AGB, N concentration in the branches and roots, and N:P ratio of the leaves but significantly decreased the BGB and root-shoot ratio ratio. Total biomass and AGB were significantly and positively correlated with leaf N and P concentrations. Leaf N concentration was significantly and positively correlated with soil available N concentration. Leaf P concentration exhibited a significant and positive correlation with soil available P. These findings provide new ideas about the effect of P addition on Chinese fir biomass accumulation and N:P stoichiometry under N deposition.