Abstract: We studied the effects of thinning on soil active organic carbon content and their proportion in total organic carbon in a 29-year-old Pinus massoniana plantation after nine years of treatment. Four thinning intensities included: light (25%, LIT), moderate (45%, MIT), high (65%, HIT), and a control (0%, CK). The results showed that compared with the CK, MIT significantly increased soil total organic carbon (TOC) content, while HIT significantly decreased TOC content. The responses of soil particulate organic carbon (POC) and readily oxidized carbon (ROC) to thinning were consistent with the TOC. The light fraction organic carbon (LFOC) content of soil was significantly decreased under the three kinds of thinning intensity in the order of HIT< LIT < MIT < CK. The distribution proportion of POC, LFOC and ROC in the TOC ranged from 30.83% to 64.59%, 10.40% to 26.92% and 3.61% to 4.62%, respectively. Thinning significantly decreased the distribution proportion of LFOC in the TOC. However, thinning had no significant effect on the distribution proportion of ROC in the TOC. MIT significantly decreased the distribution proportion of POC in the TOC. Compared to the CK, MIT was advantageous in the accumulation of TOC with enhanced stability of soil organic carbon.