Abstract: By the method of spacefortime substitution, and taking the matured (200 years old) and overmatured (>200 years old) primary broadleavedPinus koraiensis forests and their secondary forests at different succession stages (20-, 30-, 50-, 80-, and 100 years old Betula platphylla forests) in Changbai Mountains of Northeast China as test objects, this paper studied the temporal variations of soil or ganic carbon, soil microbial biomass, and soil enzyme activities during the secondary succession of primary broadleavedPinus koraiensis forests in the Mountains. Under the 20- and 80 years old B. platphylla forests, the soil organic carbon content in humus layer was the highest (154.8 and 154.3 g·kg^-1, respectively); while under the matured and overmatured primary broadleavedPinus koraiensis forests, this organic carbon content was relatively low, being 141.8 and 133.4 g·kg^-1, respectively. The soil microbial biomass carbon and microbial quotient and the activities of soil cellulase, peroxidase, acid phosphatase, and cellobiase under the 50 and  80 years old B. platphylla forests were the highest, but the activity of soil polyphenol oxidase was the lowest, which revealed that under middleaged and matured B. platphylla forests, soil organic carbon had a faster turnover rate, and was probably in a stronger accumulation phase. Statistical analysis showed that the soil microbial biomass carbon had significant positive correlations with the soil organic carbon, total nitrogen, and available phosphorus (r=0.943, 0.963, and 0.953, respectively; P<0.01).