A typical P. crassifolia forest located in the Xishui forest of the Qilian Mountains was investigated using the gradient method to study changes in carbon density in the arbor and soil layers at 2500 to 3300 m. This study provides a scientific basis for the accurate estimation of carbon storage in a Picea crassifolia forest and promotes the understanding of variation trends of organic carbon in forests under the background of climate change. The results showed that:(1) The mean biomass value of the arbor layer in the P. crassifolia forest was 115.83 t/hm², and plant carbon density was 60.23 t/hm². Biomass increased first and showed fluctuated reduction with increasing altitude, and significant differences were presented in various altitude gradients. The maximum and the minimum values appeared at altitudes of 2800 m and 3300 m separately, which were 197.10 t/hm² and 7.66 t/hm², respectively. The allocation pattern of organ biomass followed the order of stem > root > branch > leaf at all altitudes. (2) The average value of soil organic carbon (SOC) was 54.80 g/kg with variation range from 31.49 to 76.96 g/kg. Soil carbon density was negatively correlated with soil depths at all altitudes except at 3200 m and 3300 m. The maximum value (245.40 t/hm²) of soil carbon density existed at 2900 m, while the minimum value (130.24 t/hm²) appeared at 2700 m. Soil carbon density showed a gentle downward trend from 2500 m to 2700 m altitude then increased sharply at 2800 m, showed no significant fluctuations from altitudes of 2800 m to 3200 m but decreased sharply at 3300 m. (3) The mean value of carbon density in the P. crassifolia forest ecosystem was 255.15 t/hm², and the arbor layer and soil layer accounted for 23.61% and 76.39% of the whole carbon density, respectively. Whole carbon density at different altitudes showed significant differences. Soil organic carbon density was found to be positively significantly correlated with altitudinal gradient, annual precipitation, soil organic carbon content, and soil total nitrogen content, but showed obvious opposite correlations with annual temperatures in summer. Carbon density of arbor layer showed a significant positive correlation with annual temperature in summer, stand density, and basal area, but presented a significant negative correlation with soil total nitrogen. (4) Due to the influence of hydrothermal conditions, carbon density of the arbor layer and soil layer of a P. crassifolia forest along altitudinal gradients in the Qilian Mountains showed regular changes. Carbon density of P. crassifolia forest ecosystems at the altitudes of 2800-3200 m was relatively higher.