Forest carbon density is an important indicator reflecting the level of forest ecosystem function in ecological service and product supply. The methodology proposed here was used to create geographic database of forest biomass carbon storage, biomass density and carbon storage density based on forest resource investigations, and to discover the knowledge of forest carbon density distribution based on the geographic database. The geographic database of forest biomass carbon storage, biomass density and carbon storage density were created, and the knowledge of forest carbon density distribution was discovered in Shimian County by using the methodologies. There was 3.64×10⁶ton C in the forest of the county, and there was 83% in fir, spruce, hemlock and birch forest. The carbon density of the woodland was 40 t/hm², that of the spruce forest was 47 t/hm², and that of fir forest was 43 t/hm². With the increasing of slope, carbon storage increased, and with slope above 25° accounted for 91% of the total carbon storage. Carbon density in slopes above 25°was larger than that below 25°. Carbon density of the forest lands with very slight and moderate soil erosion was 42 t/hm², higher than those with severe soil erosion, say 29 t/hm². Forest carbon density significantly related to its canopy density at confidence level of 0.05. It was shown that spruce and fir forests were protected well, and of higher carbon density. The higher the carbon density, the weaker the soil erosion was there. The results played an important role in protecting forest resources, especially for provincial and national nature reserves in that county.