Extensive areas of secondary forests (SF) have been converted into larch plantations to meet the demand for timber and other forest products in northeast China since the early 1960s. However, the successive rotations of larch plantations may alter the understory vegetation and soil properties following reforestation. The aim of this study was to determine how soil carbon and nutrients and related extracellular enzymes changed after conversion from SF to larch plantations. Six larch plantations, including 16-, 30- and 47-year-old in the first rotation, 8-, 16-, and 30-year-old in the second rotation, and one adjacent SF as the 0-year site were selected using the space-for-time substitution (chronosequence) method. The results showed that most soil parameters [pH, soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN) and available potassium (AK) concentrations, and β-glucosidase, N-acetyl-glucosamine, cellobiohydrolase, and phenol oxidase activities] decreased initially from 16- to 30-year-old in the first rotation, and then increased from 30- to 47-year-old in the first rotation and 8-year-old in the second rotation, and were finally reduced from 8- to 30-year-old in the second rotation after conversion from SF, whereas soil bulk density (SBD) showed the inverse pattern. However, SOC (except at 0–10 cm) and nutrient stocks did not change significantly along the chronosequence. Redundancy analysis showed that litter biomass significantly affected most of the soil parameters, and correlated significantly and negatively with the concentrations of SOC, TN, AN and AK, and the activities of β-glucosidase, N-acetyl-glucosamine, cellobiohydrolase and phenol oxidase, while it had a significant positive relationship with SBD. Thus, rational thinning treatments should be conducted at earlier 16 years to create available space and favorable conditions for understory vegetation and litter decomposition. The appropriate rotation age of a larch plantation should be > 45 years with effective measures to maintain soil sustainability.