The elevation is a complicated environmental factor that affects the availability of light, temperature, soil nutrients, and moisture, thereby affecting the mountain ecosystem. However, it is unclear how the C:N:P ecological stoichiometry of leaf, soil, and litter changes along the altitudinal gradient, especially in the karst ecosystem in Southwest China. This work investigated the soil organic carbon (SOC), total phosphorus (TP), and total nitrogen (TN) contents, and the corresponding stoichiometric ratios of Pseudotsuga sinensis leaf, litter, and soil at an elevation range of 2127–2302 m in the northwestern Guizhou Province of China. The results indicated that altitude significantly affected the nutrient contents and stoichiometric ratios of P. sinensis in leaf, soil, and litter. The mean contents of C, N, and P in the P. sinensis forests were 473.57, 14.49, and 1.97 mg·g−1 for leaves; 422.35, 8.36, and 1.08 mg·g−1 for litter; and 40.59, 2.44, and 1.04 mg·g−1 for soil, respectively. The C, N, and P contents of soil and leaf increased significantly, whereas those in the litter decreased significantly with elevation. The average ratios of C:N, C:P, and N:P were 32.97, 244.60, and 7.42 for leaves; 51.38, 395.53, and 7.79 for litter; and 16.68, 38.84, and 2.34 for soil, respectively. The leaf C:N, N:P, and C:P ratios showed a linear decrease with an increase in altitude, whereas C:P and C:N ratios in the litter and N:P and C:P ratios in soil showed a linear increase with an increase in altitude. The resorption efficiency of both N and P of P. sinensis increased remarkably with altitude. The mean resorption efficiency of P (43.84%) was higher than that of N (40.91%). Since the N:P ratio in leaves predicts limited nutrition for plant development, we hypothesized that the N element limited the development of P. sinensis. Besides, nutrient contents were markedly related to the ratios in P. sinensis leaves, litter, and soil. The results indicate that P. sinensis had developed a favorable strategy to adapt to the high altitude and cope with unfavorable soil nutrient conditions. The N element was mainly limited to the development of P. sinensis. The P. sinensis had developed a favorable strategy to adapt to the high altitude and cope with unfavorable soil nutrient conditions. These findings highlight the relationship between the C:N:P stoichiometric ratios and altitude in plants, soils, and litters in the karst forest ecosystem, and provide insight on the better management of the karst forest ecosystems in Southwest China.