Knowledge about impacts of climatic factors on soil organic carbon (SOC) density at different scales is critical to predicting potential effect of future climate change on SOC dynamics. Based on the data of 363 forest soil profiles in Southwest China, variation of the effects of climatic factors on forest SOC density with the scale (region, province and prefecture-level city) was studied and dominant climatic factors at different scales were identified as well. Results show that the relativity of mean annual precipitation (MAP) with SOC density is decreasing with reducing scale. However, the relativity between mean annual temperature (MAT) and SOC density varies between regions, but not regularly. At the regional scale, SOC density is mainly under the joint effect of MAP and MAT. At the provincial scale, the dominant climatic factor is MAP in East Tibet, and MAT in Sichuan and Yunnan Provinces. At the city scale, similar results are found. Climatic factors can explain about 20% of the variability of SOC density at the regional scale, however, the explainability decreases with the reduction in scale.