Thinning, a common practice in forest management, has complex effects on soil organic carbon dynamics. In this study, we examined the effects of precommercial thinning on soil organic carbon (SOC) and carbon cycling-related enzyme activities in a thinning chronosequence (4–12 year recovery period) of oak-pine mixed forests in the Qinling Mountains, China. Three treatments were selected, namely, the control (CK) with no thinning, the a4 treatment (thinning 4 years ago), and the a12 treatment (thinning 12 years ago). Soil physicochemical properties, such as SOC, microbial biomass carbon (MBC), easily oxidizable organic carbon (EOC), dissolved organic carbon (DOC), and carbon cycling-related enzyme activities (hydrolase: β-glucosidase [BG], cellobiohydrolase [CBH], and invertase [INV]), were assessed. Our results indicated that thinning significantly reduced SOC by an average of 51.67% in a4 compared to CK, whereas SOC in a12 gradually returned to the unthinned level. The levels of EOC, MBC, BG, and CBH were significantly lower under the a4 and a12 treatments than under the CK treatment. Furthermore, the level of MBC was significantly higher in the a12 treatment than in the a4 treatment. There were no significant differences in DOC or INV among treatments. The structural equation model showed that thinning might regulate SOC by initially decreasing soil water content, MBC, and carbon cycling-related enzyme activities, followed by them gradually recovering to unthinned levels. These findings provide a scientific basis for understanding the response of SOC to forest thinning.