In arid regions, the conflict between forest carbon sequestration and its water consuming has severely limited regional sustainable development. Deeply understanding of the spatial distribution of vegetation carbon density for forest vegetation is essential for quantitative evaluation of forest carbon sequestration capacity and the carrying out of water-forest integrated watershed management. However, most former studies mainly focused on the comparison of carbon density among typical sample plots. The spatial distribution of carbon density for forest vegetation and its causes were still unclear. In present study, the vegetation carbon density for forest and its variation with forest structure and site conditions were analyzed based on the data of forest resources inventory in 2005 for Liupan Mountains, northwest China and the data of carbon content and the biomass regression models from published papers. The results showed that the average of carbon density for forests in Liupan Mountains was 26.7 t/hm² with a variation from 0.7 to 120.6 t/hm², of which 69.4% (8.86%-99.2%) was stored in tree layer, 25.6% (1.6%-88.1%) in shrub layer, and 4.7% (0.12%-32.3%) in herb layer. The ratio of carbon density for tree layer to the total vegetation carbon density rose with the increase of total vegetation carbon density,while the ratio for shrub layer and herb layer sharply declined. The average carbon density was 35.1 t/hm² for natural secondary forest. The average carbon density among the key natural secondary forest varied from 43.4 t/hm² for Quercus liaotungernsis forest, 35.9 t/hm² for Salix cathayana forest, 28.4 t/hm² for Betula spp. forest to 22.3 t/hm² for Populus davidiana forest. The carbon density of artifical plantation was very low with a mean value of 15.7 t/hm² (0.67-66.7 t/hm²) which was significantly lower than 35.1 t/hm² of natural secondary forest. The carbon density of forest stand increased linearly with the increase of stand age by the rate of 1.11 and 2.48 t hm^-2 a^-1 for natural secondary forest and artificial plantation, respectively. There was a tree density threshold of 1000 trees·hm^-2 and a threshold of 0.5 for canopy density of forest vegetation. When either tree density or canopy density increased beneath the values of two thresholds, the carbon density of forest stand would increase sharply. Once either tree density or canopy density of forest stand reached and exceeded these threshold values, the carbon density of forest stand would meet its maximum and increase no more. The maximum carbon density was 75.4 t/hm² and 34.6 t/hm² for natural secondary forests and plantations, respectively. The carbon density of forest stand increased sharply with the increase of annual precipitation. The maximum of carbon density for forest stand occured on the altitude of 1900 -2100 m, 2100 -2300 m, 2300 -2500 m in the areas of Liupan Mountains with annual precipitation > 700 mm, 600-700 mm, < 600 mm, respectively. It was concluded that water condition was the determinant for forest vegetation carbon density in Liupan Mountains. The canopy density of forest stands in the areas like Liupan Mountains should be kept about 0.5 for carbon sequestration.