To study the variation and response of C: N stoichiometry under different soil water and nitrogen availabilities, we used Picea asperata, the dominant species in subalpine coniferous forests of western Sichuan, China, as material to conduct the experiment. A two-factor (water and nitrogen) randomized block experiment was set up, including five water gradients (40% (W1), 50% (W2), 60% (W3), 80% (W4), and 100% (W5) of soil field capacity, respectively) and three nitrogen application concentrations (0 (N0), 20 (N1), and 40 (N2) gN m^-2 a^-1, respectively). The results showed that: (1) soil water availability and nitrogen application significantly affected C: N stoichiometry of P. asperata. Specially, C: N stoichiometry of the whole plant and organs reached the highest value under N0W4 treatment, which decreased with the decline of soil water availability as well as the increase of nitrogen application. (2) with the decrease of soil water availability, the carbon concentration of roots and leaves significantly increased, whereas, those of stem and leaves showed a decline with the increase of nitrogen application. In addition, the decline of soil water availability markedly promoted the nitrogen concentration of roots and stem, and nitrogen concentration of organs increased gradually with more nitrogen application. The carbon and nitrogen concentration decreased consistently in the order of leaves > stem > roots and leaves > roots > stem under the same treatment. (3) the net photosynthetic rate of P. asperata rose first and then decreased with the decline of soil water availability, and increased with more nitrogen application, which reached the maximum value under N2W4 treatment. (4) net fluxes of NH₄⁺ and NO₃^- in roots decreased obviously with the decline of soil water availability, and increased with more nitrogen application. besides, net flux of NH₄⁺ in roots was correlated negatively with soil available nitrogen concentration. The results showed that soil water and nitrogen availabilities affected carbon assimilation and nitrogen uptake processes of P. asperata, and changed the nutrient utilization efficiency and carbon and nitrogen concentration, which resulted in changes of C: N stoichiometry.