Thinning management is used to improve timber production, but only a few data are available on how it influences ecosystem C sink capacity. This study aims to clarify the effects of thinning on C sinks of larch plantations, the most widespread forests in Northeastern China. Both C influx from biomass production and C efflux from each soil respiration component and its temperature sensitivity were determined for scaling-up ecosystem C sink estimation: microbial composition is measured for clarifying mechanism for respiratory changes from thinning treatment. Thinning management induced 6.23 mol C m⁻² yr⁻¹ increase in biomass C, while the decrease in heterotrophic respiration (R(h)) at the thinned sites (0.9 mol C m⁻² yr⁻¹) has enhanced 14% of this biomass C increase. This decrease in R(h) was a sum of the 42% decrease (4.1 mol C m⁻² yr⁻¹) in litter respiration and 3.2 mol C m⁻² yr⁻¹ more CO₂ efflux from mineral soil in thinned sites compared with unthinned control. Increases in temperature, temperature sensitivity, alteration of litters, and microbial composition may be responsible for the contrary changes in R h from mineral soil and litter respiration, respectively. These findings manifested that thinning management of larch plantations could enhance biomass accumulation and decrease respiratory efflux from soil, which resulted in the effectiveness improvement in sequestrating C in forest ecosystems.