Coarse woody debris (CWD) is a substantial carbon pool and a source of nutrients within forest ecosystems. It plays several important ecological roles including the regulation of nutrient cycling and energy flow, as well as habitat provision for many organisms. The CWD carbon density basic character, the relationship between CWD carbon density and stand factors, and species diversity were studied in a large permanent plot (9.12 hm²) of spruce-fir valley forest in Xiaoxing'an Mountains. The results showed that: (1) total carbon density of CWD in the spruce-fir valley forest was 13.25 tC/hm², and the carbon density of CWD for spruce (Picea spp.), fir (Abies nephrolepis), dahurian larch (Larix gmelinii), and unidentified species were 3.59, 2.61, 3.06, and 2.85 tC/hm², respectively. (2) CWD carbon density under different decay classes followed a normal distribution with greater concentrations in classes II and III, which accounted for 42.7% and 35.4% of the total CWD carbon density, respectively. CWD carbon density within different DBH classes also followed a normal distribution, with higher values in the 30-40 cm and 40-50 cm classes. The primary forms of CWD carbon density originated from breakage at the trunk, uprooted blow-down, standing dead, and breakage at the rootstock. Uprooted blow-down had the highest carbon density in decay classes I and V. However, breakage at the trunk had the highest carbon density of other decay classes. (3) CWD carbon density showed a high spatial heterogeneity and it decreased with increasing stand mean DBH, maximum DBH, and basal area (P < 0.05). No significant relationships were observed between CWD carbon density and stand density, diversity index, and evenness index.