Surface soil samples (0~20cm) were collected from a Calamagrostis angustifolia freshwater marsh in the Sanjiang Plain from May to September and soil active organic carbon including microbial biomass carbon (MBC), easily oxidizable carbon (EOC), dissolved organic carbon (DOC) were analyzed aiming to reveal their seasonal dynamics characterisitics and their correlationships with activities of soil invertase, amylase and cellulase. The results indicated that MBC, EOC and DOC varied significantly with season and their variation coefficients were 42.1%, 16.6%, 7.7% respectively. MBC increased slowly from may to June and then decreased to minimum in July, then increased rapidly and occurred maximum in September. EOC and DOC had similar dynamic rules, they decreased from May to July, then they increased rapidly, and the content of EOC and DOC in July were minimum and that in September were maximum. And the seasonal dynamics of the ratios of MBC, EOC and DOC to SOC were similar to MBC, EOC and DOC. Soil invertase, amylase and cellulase are key enzymes involved soil carbon cycle. The activities of invertase and cellulase were similar to MBC with seasonal variation. The seasonal change of amylase activity was different from the other two enzyme activities, which firstly decreased rapidly from May to July, and then increased gradually and maximum value of amylase activity was in September. Correlation analysis suggested that soil organic carbon fractions had significant response to the activities of soil invertase, amylase and cellulase with the variation of season. Invertase activity was significantly positively related to SOC, MBC, EOC and DOC (p<;0.05), amylase activity was significantly positive correlation with EOC and DOC (p<;0.05), and cellulase activity was significantly positive correlation with SOC and MBC (p<;0.05).